netfilter: do not propagate nf_queue errors in nf_hook_slow

commit f1585086
(netfilter: nfnetlink_queue: return error number to caller)
erronously assigns the return value of nf_queue() to the "ret" value.

This can cause bogus return values if we encounter QUEUE verdict
when bypassing is enabled, the listener does not exist and the
next hook returns NF_STOLEN.

In this case nf_hook_slow returned -ESRCH instead of 0.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>