netfilter: nfnetlink: silence warning if CONFIG_PROVE_RCU isn't set

Since commit c14b78e7 ("netfilter:
nfnetlink: add mutex per subsystem") building nefnetlink.o without
CONFIG_PROVE_RCU set, triggers this GCC warning:
    net/netfilter/nfnetlink.c:65:22: warning: ‘nfnl_get_lock’ defined but not used [-Wunused-function]

The cause of that warning is, in short, that rcu_lockdep_assert()
compiles away if CONFIG_PROVE_RCU is not set. Silence this warning by
open coding nfnl_get_lock() in the sole place it was called, which
allows to remove that function.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>