ipv4 : igmp : fix error handle in ip_mc_add_src()

When add sources to interface failure, need to roll back the sfcount[MODE]
to before state. We need to match it corresponding.
Acked-by: NDavid L Stevens <dlstevens@us.ibm.com>
Acked-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NJun Zhao <mypopydev@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -1716,7 +1716,8 @@ static int ip_mc_add_src(struct in_device *in_dev, __be32 *pmca, int sfmode,
 	if (err) {
 		int j;
 
-		pmc->sfcount[sfmode]--;
+		if (!delta)
+			pmc->sfcount[sfmode]--;
 		for (j=0; j<i; j++)
 			(void) ip_mc_del1_src(pmc, sfmode, &psfsrc[j]);
 	} else if (isexclude != (pmc->sfcount[MCAST_EXCLUDE] != 0)) {