t_msgsnd: Use msgsnd()'s msgsz argument correctly to avoid overflow

msgsnd's msgsz argument is the size of the message following the 'long'
message type.  Don't include the message type in the size of the message
when invoking msgsnd(2).

Reported by:	Coverity
CID:		1368712
Sponsored by:	Dell EMC Isilon

contrib/netbsd-tests/lib/libc/sys/t_msgsnd.c

@@ -234,7 +234,7 @@ ATF_TC_BODY(msgsnd_nonblock, tc)
 		for (;;) {
 
 			errno = 0;
-			rv = msgsnd(id, &msg, sizeof(struct msg), IPC_NOWAIT);
+			rv = msgsnd(id, &msg, sizeof(msg.buf), IPC_NOWAIT);
 
 			if (rv == -1 && errno == EAGAIN)
 				_exit(EXIT_SUCCESS);