source-nfq: fix tunnel mark callback algorithm

In case of a tunnel packet, adding a mark to the root packet will have
for consequence to bypass all the flows that are hosted in this tunnel.
This is not the attended behavior and as initial fix let's simply warn
suricata that bypass for NFQ is not possible for this kind of packets.

This patch also fixes a segfault. The root packet was accessed even if it is
NULL causing a NULL dereference:

ASAN:SIGSEGV
=================================================================
==24408==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000060 (pc 0x00000076f948 bp 0x7f435c000240 sp 0x7f435c000220 T5)
ASAN:SIGSEGV
==24408==AddressSanitizer: while reporting a bug found another one. Ignoring.
    #0 0x76f947 in NFQBypassCallback /home/victor/dev/suricata/src/source-nfq.c:510
    #1 0x4d0f02 in PacketBypassCallback /home/victor/dev/suricata/src/decode.c:395
    #2 0x7b8a95 in StreamTcpPacket /home/victor/dev/suricata/src/stream-tcp.c:4661
    #3 0x7b9ddd in StreamTcp /home/victor/dev/suricata/src/stream-tcp.c:4913
    #4 0x68fa50 in FlowWorker /home/victor/dev/suricata/src/flow-worker.c:194
    #5 0x7f0abd in TmThreadsSlotVarRun /home/victor/dev/suricata/src/tm-threads.c:128
    #6 0x7f2958 in TmThreadsSlotVar /home/victor/dev/suricata/src/tm-threads.c:585
    #7 0x7f436368e6f9 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x76f9)
    #8 0x7f4362802b5c in clone (/lib/x86_64-linux-gnu/libc.so.6+0x106b5c)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/victor/dev/suricata/src/source-nfq.c:510 NFQBypassCallback
Thread T5 (W#04) created by T0 (Suricata-Main) here:
    #0 0x7f4364ff2253 in pthread_create (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x36253)
    #1 0x7f9c48 in TmThreadSpawn /home/victor/dev/suricata/src/tm-threads.c:1843
    #2 0x8da7c0 in RunModeSetIPSAutoFp /home/victor/dev/suricata/src/util-runmodes.c:519
    #3 0x73e3ff in RunModeIpsNFQAutoFp /home/victor/dev/suricata/src/runmode-nfq.c:74
    #4 0x7503fa in RunModeDispatch /home/victor/dev/suricata/src/runmodes.c:382
    #5 0x7e5cb3 in main /home/victor/dev/suricata/src/suricata.c:2547
    #6 0x7f436271c82f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

src/source-nfq.c

@@ -504,14 +504,7 @@ static void NFQReleasePacket(Packet *p)
 static int NFQBypassCallback(Packet *p)
 {
     if (IS_TUNNEL_PKT(p)) {
-        SCMutex *m = p->root ? &p->root->tunnel_mutex : &p->tunnel_mutex;
-        SCMutexLock(m);
-
-        p->root->nfq_v.mark = (nfq_config.bypass_mark & nfq_config.bypass_mask)
-                        | (p->nfq_v.mark & ~nfq_config.bypass_mask);
-        p->root->flags |= PKT_MARK_MODIFIED;
-
-        SCMutexUnlock(m);
+        return 0;
     } else {
         p->nfq_v.mark = (nfq_config.bypass_mark & nfq_config.bypass_mask)
                         | (p->nfq_v.mark & ~nfq_config.bypass_mask);