[NETFILTER] ctnetlink: propagate error instaed of returning -EPERM

Propagate the error to userspace instead of returning -EPERM if the get
conntrack operation fails.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>