netfilter: nf_log_syslog: Consolidate entry checks

Every syslog logging callback has to perform the same check to cover for
rogue containers, introduce a helper for clarity. Drop the FIXME as
there is a viable solution since commit 2851940f

 ("netfilter: allow
logging from non-init namespaces").

Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>