ip_output, KERNEL_LOCK and multicast

[Martin Pieuchot]

Do not grab the KERNEL_LOCK if the source address is already specified.

Note that the sending path will still be done while holding the
KERNEL_LOCK but it doesn't hurt to get this right.  Plus this makes it
similar to another chunk in this file.

ok?

Index: netinet/ip_output.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_output.c,v
retrieving revision 1.322
diff -u -p -r1.322 ip_output.c
--- netinet/ip_output.c 4 May 2016 13:22:51 -0000       1.322
+++ netinet/ip_output.c 30 May 2016 04:53:09 -0000
@@ -185,14 +185,17 @@ reroute:
        if ((IN_MULTICAST(ip->ip_dst.s_addr) ||
            (ip->ip_dst.s_addr == INADDR_BROADCAST)) &&
            imo != NULL && (ifp = if_get(imo->imo_ifidx)) != NULL) {
-               struct in_ifaddr *ia;
 
                mtu = ifp->if_mtu;
-               KERNEL_LOCK();
-               IFP_TO_IA(ifp, ia);
-               if (ip->ip_src.s_addr == INADDR_ANY && ia)
-                       ip->ip_src = ia->ia_addr.sin_addr;
-               KERNEL_UNLOCK();
+               if (ip->ip_src.s_addr == INADDR_ANY) {
+                       struct in_ifaddr *ia;
+
+                       KERNEL_LOCK();
+                       IFP_TO_IA(ifp, ia);
+                       if (ia != NULL)
+                               ip->ip_src = ia->ia_addr.sin_addr;
+                       KERNEL_UNLOCK();
+               }
        } else {
                struct in_ifaddr *ia;