netfilter: nf_nat: missing condition in nf_xfrm_me_harder()

This if statement was accidentally dropped in (aaa795ad netfilter:
nat: propagate errors from xfrm_me_harder()) so now it returns
unconditionally.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>