xfs: convert XFS_AGFL_SIZE to a helper function

The AGFL size calculation is about to get more complex, so lets turn
the macro into a function first and remove the macro.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[darrick: forward port to newer kernel, simplify the helper]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>