core: work around interface enum buffer overrun

When a system has too many interfaces and too many addresses,
8 kilobytes isn't enough to fit all of the netlink responses.
As the result, kamailio gets stuck in a loop where it tries
to do a 0-length recv.

Increase the buffer to 32K. It's a miniscule amount for modern
times anyway. Also, add diagnostics to make further troubleshooting
easier.

Proper fix would be to switch to libnl here, which would make a
good weekend project.

(cherry picked from commit d2fd204)

src/core/socket_info.c

@@ -1070,6 +1070,7 @@ static int nl_bound_sock(void)
 	req.g.rtgen_family = family;\
 	} while(0);
 
+#define NETLINK_BUFFER_SIZE 32768
 
 static int get_flags(int family){
 	struct {
@@ -1079,7 +1080,7 @@ static int get_flags(int family){
 	int rtn = 0;
 	struct nlmsghdr*  nlp;
 	struct ifinfomsg *ifi;
-	char buf[8192];
+	char buf[NETLINK_BUFFER_SIZE];
 	char *p = buf;
 	int nll = 0;
     int nl_sock = -1;
@@ -1095,6 +1096,10 @@ static int get_flags(int family){
 	}
 
 	while(1) {
+		if ((sizeof(buf) - nll) == 0) {
+			LM_ERR("netlink buffer overflow in get_flags");
+			goto error;
+		}
 		rtn = recv(nl_sock, p, sizeof(buf) - nll, 0);
 		nlp = (struct nlmsghdr *) p;
 		if(nlp->nlmsg_type == NLMSG_DONE){
@@ -1148,7 +1153,7 @@ static int build_iface_list(void)
 	struct nlmsghdr*  nlp;
 	struct ifaddrmsg *ifi;
 	int rtl;
-	char buf[8192];
+	char buf[NETLINK_BUFFER_SIZE];
 	char *p = buf;
 	int nll = 0;
 	struct rtattr * rtap;
@@ -1184,6 +1189,10 @@ static int build_iface_list(void)
 		nll = 0;
 		p = buf;
 		while(1) {
+			if ((sizeof(buf) - nll) == 0) {
+				LM_ERR("netlink buffer overflow in build_iface_list");
+				goto error;
+			}
 			rtn = recv(nl_sock, p, sizeof(buf) - nll, 0);
 			LM_DBG("received %d byles \n", rtn);
 			nlp = (struct nlmsghdr *) p;