xfs: factor out filestreams from xfs_bmap_btalloc_nullfb

There's many if (filestreams) {} else {} branches in this function.
Split it out into a filestreams specific function so that we can
then work directly on cleaning up the filestreams code without
impacting the rest of the allocation algorithms.

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