libxfs: resync with the userspace libxfs

Prepare to resync the userspace libxfs with the kernel libxfs.  There
were a few things I missed -- a couple of static inline directory
functions that have to be exported for xfs_repair; a couple of directory
naming functions that make porting much easier if they're /not/ static
inline; and a u16 usage that should have been uint16_t.

None of these things are bugs in their own right; this just makes
porting xfsprogs easier.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>