xfs: add xfs sb v4 support for dirent filetype field

Add XFS superblock v4 support for the file type field in the
directory entry feature.

This support adds a feature bit for version 4 superblocks and
leaves the original superblock 5 incompatibility bit.

Signed-off-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Geoffrey Wehrman <gwehrman@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>