Remove zpl_revalidate: fix snapshot rollback

Open files, which aren't present in the snapshot, which is being
roll-backed to, need to disappear from the visible VFS image of
the dataset.

Kernel provides d_drop function to drop invalid entry from
the dcache, but inode can be referenced by dentry multiple dentries.

The introduced zpl_d_drop_aliases function walks and invalidates
all aliases of an inode.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Closes #9600
Closes #14070

include/sys/zfs_znode.h

@@ -190,7 +190,6 @@ typedef struct znode {
 	boolean_t	z_is_sa;	/* are we native sa? */
 	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? */
 	boolean_t	z_suspended;	/* extra ref from a suspend? */
 	uint_t		z_blksz;	/* block size in bytes */
 	uint_t		z_seq;		/* modification sequence number */