netfilter: nf_tables: avoid false-postive lockdep splat

There are reports wrt lockdep splat in nftables, e.g.:
------------[ cut here ]------------
WARNING: CPU: 2 PID: 31416 at net/netfilter/nf_tables_api.c:622
lockdep_nfnl_nft_mutex_not_held+0x28/0x38 [nf_tables]
...

These are caused by an earlier, unrelated bug such as a n ABBA deadlock
in a different subsystem.
In such an event, lockdep is disabled and lockdep_is_held returns true
unconditionally.  This then causes the WARN() in nf_tables.

Make the WARN conditional on lockdep still active to avoid this.

Fixes: f102d66b ("netfilter: nf_tables: use dedicated mutex to guard transactions")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://lore.kernel.org/linux-kselftest/CA+G9fYvFUpODs+NkSYcnwKnXm62tmP=ksLeBPmB+KFrB2rvCtQ@mail.gmail.com/Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_tables_api.c

@@ -619,6 +619,7 @@ static int nft_request_module(struct net *net, const char *fmt, ...)
 static void lockdep_nfnl_nft_mutex_not_held(void)
 {
 #ifdef CONFIG_PROVE_LOCKING
+	if (debug_locks)
 		WARN_ON_ONCE(lockdep_nfnl_is_held(NFNL_SUBSYS_NFTABLES));
 #endif
 }