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: NMark Tinguely <tinguely@sgi.com>
Reviewed-by: NGeoffrey Wehrman <gwehrman@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>