xfs_scrub: add file space map iteration functions

These helpers enable userspace to iterate all the space map information
for a file.  The iteration function uses GETBMAPX.

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