pipe: Check for ring full inside of the spinlock in pipe_write()

Make pipe_write() check to see if the ring has become full between it
taking the pipe mutex, checking the ring status and then taking the
spinlock.

This can happen if a notification is written into the pipe as that happens
without the pipe mutex.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/pipe.c

@@ -463,6 +463,11 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			spin_lock_irq(&pipe->wait.lock);
 
 			head = pipe->head;
+			if (pipe_full(head, pipe->tail, max_usage)) {
+				spin_unlock_irq(&pipe->wait.lock);
+				continue;
+			}
+
 			pipe->head = head + 1;
 
 			/* Always wake up, even if the copy fails. Otherwise