[XFS] Remove vn_revalidate calls in xfs.

These days most of the attributes in struct inode are properly kept in
sync by XFS. This patch removes the need for vn_revalidate completely by:

- keeping inode.i_flags uptodate after any flags are updated in

xfs_ioctl_setattr

- keeping i_mode, i_uid and i_gid uptodate in xfs_setattr

SGI-PV: 984566

SGI-Modid: xfs-linux-melb:xfs-kern:31679a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>