xfs: refactor agfl length computation function

commit 1cac233c upstream.

Refactor xfs_alloc_min_freelist to accept a NULL @pag argument, in which
case it returns the largest possible minimum length.  This will be used
in an upcoming patch to compute the length of the AGFL at mkfs time.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>