nbd: pass MSG_* flags to kernel_recvmsg()

Unlike kernel_sendmsg(), kernel_recvmsg() requires passing flags explicitly
via last parameter instead of struct msghdr.msg_flags. Therefore calls to
sock_xmit(lo, 0, ..., MSG_WAITALL) have not been processed properly by tcp
layer wrt. the flag. Fix it.
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Clements <Paul.Clements@steeleye.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/nbd.c

@@ -192,7 +192,8 @@ static int sock_xmit(struct nbd_device *lo, int send, void *buf, int size,
 			if (lo->xmit_timeout)
 				del_timer_sync(&ti);
 		} else
-			result = kernel_recvmsg(sock, &msg, &iov, 1, size, 0);
+			result = kernel_recvmsg(sock, &msg, &iov, 1, size,
+						msg.msg_flags);
 
 		if (signal_pending(current)) {
 			siginfo_t info;