Acquire Giant arounds calls into the linker from Netgraph sockets.

We now no longer hold Giant in send(), so it isn't inheritted by the
linker, which calls into VFS.

Reported by:	glebius
Discussed with:	glebius, bz

sys/netgraph/ng_socket.c

@@ -300,7 +300,9 @@ printf("errx=%d\n",error);
 
 			/* Not found, try to load it as a loadable module */
 			snprintf(filename, sizeof(filename), "ng_%s", mkp->type);
+			mtx_lock(&Giant);
 			error = linker_load_module(NULL, filename, NULL, NULL, &lf);
+			mtx_unlock(&Giant);
 			if (error != 0) {
 				FREE(msg, M_NETGRAPH_MSG);
 				goto release;