netfilter: nf_log: wait for rcu grace after logger unregistration

The nf_log_unregister() function needs to call synchronize_rcu() to make sure
that the objects are not dereferenced anymore on module removal.

Fixes: 5962815a ("netfilter: nf_log: use an array of loggers instead of list")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>