xfs: use xfs_bmap_longest_free_extent() in filestreams

The code in xfs_bmap_longest_free_extent() is open coded in
xfs_filestream_pick_ag(). Export xfs_bmap_longest_free_extent and
call it from the filestreams code instead.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>