xfs: always defer agfl block frees

The AGFL fixup code conditionally defers block frees from the free
list based on whether the current transaction has an associated
xfs_defer_ops structure. Now that dfops is embedded in the
transaction and the internal dfops is used unconditionally, this
invariant is always true.

Remove the now dead logic to check for ->t_dfops in
xfs_alloc_fix_freelist() and unconditionally defer AGFL block frees.

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