xfs: refactor allocation tree fixup code

Both algorithms duplicate the same btree allocation code. Eliminate
the duplication and reuse the fallback algorithm codepath.

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