xfs: hoist xfs_scrub_agfl_walk to libxfs as xfs_agfl_walk

This function is basically a generic AGFL block iterator, so promote it
to libxfs ahead of online repair wanting to use it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>