xfs: clean up small allocation helper

xfs_alloc_ag_vextent_small() is kind of a mess. Clean it up in
preparation for future changes. No functional changes.

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