[XFS] Don't update i_size for directories and special files

The core kernel uses vfs_getattr to look at the inode size and similar
attributes, so there is no need to keep i_size uptodate for directories or
special files. This means we can remove xfs_validate_fields because the
I/O path already keeps i_size uptodate for regular files.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31336a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>