Fix 'zfs rollback' on mounted file systems

Rolling back a mounted filesystem with open file handles and
cached dentries+inodes never worked properly in ZoL.  The
major issue was that Linux provides no easy mechanism for
modules to invalidate the inode cache for a file system.

Because of this it was possible that an inode from the previous
filesystem would not get properly dropped from the cache during
rolling back.  Then a new inode with the same inode number would
be create and collide with the existing cached inode.  Ideally
this would trigger an VERIFY() but in practice the error wasn't
handled and it would just NULL reference.

Luckily, this issue can be resolved by sprucing up the existing
Solaris zfs_rezget() functionality for the Linux VFS.

The way it works now is that when a file system is rolled back
all the cached inodes will be traversed and refetched from disk.
If a version of the cached inode exists on disk the in-core
copy will be updated accordingly.  If there is no match for that
object on disk it will be unhashed from the inode cache and
marked as stale.

This will effectively make the inode unfindable for lookups
allowing the inode number to be immediately recycled.  The inode
will then only be accessible from the cached dentries.  Subsequent
dentry lookups which reference a stale inode will result in the
dentry being invalidated.  Once invalidated the dentry will drop
its reference on the inode allowing it to be safely pruned from
the cache.

Special care is taken for negative dentries since they do not
reference any inode.  These dentires will be invalidate based
on when they were added to the dentry cache.  Entries added
before the last rollback will be invalidate to prevent them
from masking real files in the dataset.

Two nice side effects of this fix are:

* Removes the dependency on spl_invalidate_inodes(), it can now
  be safely removed from the SPL when we choose to do so.

* zfs_znode_alloc() no longer requires a dentry to be passed.
  This effectively reverts this portition of the code to its
  upstream counterpart.  The dentry is not instantiated more
  correctly in the Linux ZPL layer.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #795

include/sys/zfs_vfsops.h

@@ -69,6 +69,7 @@ typedef struct zfs_sb {
 	krwlock_t	z_teardown_inactive_lock;
 	list_t		z_all_znodes;	/* all znodes in the fs */
 	uint64_t	z_nr_znodes;	/* number of znodes in the fs */
+	unsigned long	z_rollback_time;/* last online rollback time */
 	kmutex_t	z_znodes_lock;	/* lock for z_all_znodes */
 	struct inode	*z_ctldir;	/* .zfs directory inode */
 	avl_tree_t	z_ctldir_snaps;	/* .zfs/snapshot entries */

include/sys/zfs_znode.h

@@ -216,6 +216,7 @@ typedef struct znode {
 	boolean_t	z_is_zvol;	/* are we used by the zvol */
 	boolean_t	z_is_mapped;	/* are we mmap'ed */
 	boolean_t	z_is_ctldir;	/* are we .zfs entry */
+	boolean_t	z_is_stale;	/* are we stale due to rollback? */
 	struct inode	z_inode;	/* generic vfs inode */
 } znode_t;
 

include/sys/zpl.h

@@ -28,18 +28,20 @@
 #include <sys/vfs.h>
 #include <linux/vfs_compat.h>
 #include <linux/xattr_compat.h>
+#include <linux/dcache_compat.h>
 #include <linux/exportfs.h>
 #include <linux/writeback.h>
 #include <linux/falloc.h>
 
 /* zpl_inode.c */
 extern void zpl_vap_init(vattr_t *vap, struct inode *dir,
-    struct dentry *dentry, zpl_umode_t mode, cred_t *cr);
+    zpl_umode_t mode, cred_t *cr);
 
 extern const struct inode_operations zpl_inode_operations;
 extern const struct inode_operations zpl_dir_inode_operations;
 extern const struct inode_operations zpl_symlink_inode_operations;
 extern const struct inode_operations zpl_special_inode_operations;
+extern dentry_operations_t zpl_dentry_operations;
 
 /* zpl_file.c */
 extern ssize_t zpl_read_common(struct inode *ip, const char *buf,