Linux compat: Minimum kernel version 3.10

Increase the minimum supported kernel version from 2.6.32 to 3.10.
This removes support for the following Linux enterprise distributions.

    Distribution     | Kernel | End of Life
    ---------------- | ------ | -------------
    Ubuntu 12.04 LTS | 3.2    | Apr 28, 2017
    SLES 11          | 3.0    | Mar 32, 2019
    RHEL / CentOS 6  | 2.6.32 | Nov 30, 2020

The following changes were made as part of removing support.

* Updated `configure` to enforce a minimum kernel version as
  specified in the META file (Linux-Minimum: 3.10).

    configure: error:
        *** Cannot build against kernel version 2.6.32.
        *** The minimum supported kernel version is 3.10.

* Removed all `configure` kABI checks and matching C code for
  interfaces which solely predate the Linux 3.10 kernel.

* Updated all `configure` kABI checks to fail when an interface is
  missing which was in the 3.10 kernel up to the latest 5.1 kernel.
  Removed the HAVE_* preprocessor defines for these checks and
  updated the code to unconditionally use the verified interface.

* Inverted the detection logic in several kABI checks to match
  the new interface as it appears in 3.10 and newer and not the
  legacy interface.

* Consolidated the following checks in to individual files. Due
  the large number of changes in the checks it made sense to handle
  this now.  It would be desirable to group other related checks in
  the same fashion, but this as left as future work.

  - config/kernel-blkdev.m4 - Block device kABI checks
  - config/kernel-blk-queue.m4 - Block queue kABI checks
  - config/kernel-bio.m4 - Bio interface kABI checks

* Removed the kABI checks for sops->nr_cached_objects() and
  sops->free_cached_objects().  These interfaces are currently unused.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9566

module/os/linux/zfs/zpl_ctldir.c

@@ -114,11 +114,7 @@ zpl_root_getattr_impl(const struct path *path, struct kstat *stat,
 ZPL_GETATTR_WRAPPER(zpl_root_getattr);
 
 static struct dentry *
-#ifdef HAVE_LOOKUP_NAMEIDATA
-zpl_root_lookup(struct inode *dip, struct dentry *dentry, struct nameidata *nd)
-#else
 zpl_root_lookup(struct inode *dip, struct dentry *dentry, unsigned int flags)
-#endif
 {
 	cred_t *cr = CRED();
 	struct inode *ip;
@@ -160,7 +156,6 @@ const struct inode_operations zpl_ops_root = {
 	.getattr	= zpl_root_getattr,
 };
 
-#ifdef HAVE_AUTOMOUNT
 static struct vfsmount *
 zpl_snapdir_automount(struct path *path)
 {
@@ -179,7 +174,6 @@ zpl_snapdir_automount(struct path *path)
 	 */
 	return (NULL);
 }
-#endif /* HAVE_AUTOMOUNT */
 
 /*
  * Negative dentries must always be revalidated so newly created snapshots
@@ -206,21 +200,13 @@ dentry_operations_t zpl_dops_snapdirs = {
  * name space.  While it might be possible to add compatibility
  * code to accomplish this it would require considerable care.
  */
-#ifdef HAVE_AUTOMOUNT
 	.d_automount	= zpl_snapdir_automount,
-#endif /* HAVE_AUTOMOUNT */
 	.d_revalidate	= zpl_snapdir_revalidate,
 };
 
 static struct dentry *
-#ifdef HAVE_LOOKUP_NAMEIDATA
-zpl_snapdir_lookup(struct inode *dip, struct dentry *dentry,
-    struct nameidata *nd)
-#else
 zpl_snapdir_lookup(struct inode *dip, struct dentry *dentry,
     unsigned int flags)
-#endif
-
 {
 	fstrans_cookie_t cookie;
 	cred_t *cr = CRED();
@@ -241,9 +227,7 @@ zpl_snapdir_lookup(struct inode *dip, struct dentry *dentry,
 	ASSERT(error == 0 || ip == NULL);
 	d_clear_d_op(dentry);
 	d_set_d_op(dentry, &zpl_dops_snapdirs);
-#ifdef HAVE_AUTOMOUNT
 	dentry->d_flags |= DCACHE_NEED_AUTOMOUNT;
-#endif
 
 	return (d_splice_alias(ip, dentry));
 }
@@ -348,7 +332,7 @@ zpl_snapdir_rmdir(struct inode *dip, struct dentry *dentry)
 }
 
 static int
-zpl_snapdir_mkdir(struct inode *dip, struct dentry *dentry, zpl_umode_t mode)
+zpl_snapdir_mkdir(struct inode *dip, struct dentry *dentry, umode_t mode)
 {
 	cred_t *cr = CRED();
 	vattr_t *vap;
@@ -433,13 +417,8 @@ const struct inode_operations zpl_ops_snapdir = {
 };
 
 static struct dentry *
-#ifdef HAVE_LOOKUP_NAMEIDATA
-zpl_shares_lookup(struct inode *dip, struct dentry *dentry,
-    struct nameidata *nd)
-#else
 zpl_shares_lookup(struct inode *dip, struct dentry *dentry,
     unsigned int flags)
-#endif
 {
 	fstrans_cookie_t cookie;
 	cred_t *cr = CRED();