Add .zfs control directory

Add support for the .zfs control directory.  This was accomplished
by leveraging as much of the existing ZFS infrastructure as posible
and updating it for Linux as required.  The bulk of the core
functionality is now all there with the following limitations.

*) The .zfs/snapshot directory automount support requires a 2.6.37
   or newer kernel.  The exception is RHEL6.2 which has backported
   the d_automount patches.

*) Creating/destroying/renaming snapshots with mkdir/rmdir/mv
   in the .zfs/snapshot directory works as expected.  However,
   this functionality is only available to root until zfs
   delegations are finished.

      * mkdir - create a snapshot
      * rmdir - destroy a snapshot
      * mv    - rename a snapshot

The following issues are known defeciences, but we expect them to
be addressed by future commits.

*) Add automount support for kernels older the 2.6.37.  This should
   be possible using follow_link() which is what Linux did before.

*) Accessing the .zfs/snapshot directory via NFS is not yet possible.
   The majority of the ground work for this is complete.  However,
   finishing this work will require resolving some lingering
   integration issues with the Linux NFS kernel server.

*) The .zfs/shares directory exists but no futher smb functionality
   has yet been implemented.

Contributions-by: Rohan Puri <rohan.puri15@gmail.com>
Contributiobs-by: Andrew Barnes <barnes333@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #173

module/zfs/zpl_inode.c

@@ -25,6 +25,7 @@
 
 #include <sys/zfs_vfsops.h>
 #include <sys/zfs_vnops.h>
+#include <sys/zfs_znode.h>
 #include <sys/vfs.h>
 #include <sys/zpl.h>
 
@@ -51,7 +52,7 @@ zpl_lookup(struct inode *dir, struct dentry *dentry, struct nameidata *nd)
 	return d_splice_alias(ip, dentry);
 }
 
-static void
+void
 zpl_vap_init(vattr_t *vap, struct inode *dir, struct dentry *dentry,
     mode_t mode, cred_t *cr)
 {
@@ -171,8 +172,20 @@ zpl_rmdir(struct inode * dir, struct dentry *dentry)
 static int
 zpl_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
 {
+	boolean_t issnap = ITOZSB(dentry->d_inode)->z_issnap;
 	int error;
 
+	/*
+	 * Ensure MNT_SHRINKABLE is set on snapshots to ensure they are
+	 * unmounted automatically with the parent file system.  This
+	 * is done on the first getattr because it's not easy to get the
+	 * vfsmount structure at mount time.  This call path is explicitly
+	 * marked unlikely to avoid any performance impact.  FWIW, ext4
+	 * resorts to a similar trick for sysadmin convenience.
+	 */
+	if (unlikely(issnap && !(mnt->mnt_flags & MNT_SHRINKABLE)))
+		mnt->mnt_flags |= MNT_SHRINKABLE;
+
 	error = -zfs_getattr_fast(dentry->d_inode, stat);
 	ASSERT3S(error, <=, 0);