netfilter: nf_nat_sip: fix incorrect handling of EBUSY for RTCP expectation

We're hitting bug while trying to reinsert an already existing
expectation:

kernel BUG at kernel/timer.c:895!
invalid opcode: 0000 [#1] SMP
[...]
Call Trace:
 <IRQ>
 [<ffffffffa0069563>] nf_ct_expect_related_report+0x4a0/0x57a [nf_conntrack]
 [<ffffffff812d423a>] ? in4_pton+0x72/0x131
 [<ffffffffa00ca69e>] ip_nat_sdp_media+0xeb/0x185 [nf_nat_sip]
 [<ffffffffa00b5b9b>] set_expected_rtp_rtcp+0x32d/0x39b [nf_conntrack_sip]
 [<ffffffffa00b5f15>] process_sdp+0x30c/0x3ec [nf_conntrack_sip]
 [<ffffffff8103f1eb>] ? irq_exit+0x9a/0x9c
 [<ffffffffa00ca738>] ? ip_nat_sdp_media+0x185/0x185 [nf_nat_sip]

We have to remove the RTP expectation if the RTCP expectation hits EBUSY
since we keep trying with other ports until we succeed.
Reported-by: Rafal Fitt <rafalf@aplusc.com.pl>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_nat_sip.c

@@ -502,7 +502,10 @@ static unsigned int ip_nat_sdp_media(struct sk_buff *skb, unsigned int dataoff,
 		ret = nf_ct_expect_related(rtcp_exp);
 		if (ret == 0)
 			break;
-		else if (ret != -EBUSY) {
+		else if (ret == -EBUSY) {
+			nf_ct_unexpect_related(rtp_exp);
+			continue;
+		} else if (ret < 0) {
 			nf_ct_unexpect_related(rtp_exp);
 			port = 0;
 			break;