xfs: move small allocation helper

Move the small allocation helper further up in the file to avoid the
need for a function declaration. The remaining declarations will be
removed by followup patches. No functional changes.

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>