pipe: fix incorrect caching of pipe state over pipe_wait()

Similarly to commit 8f868d68d335 ("pipe: Fix missing mask update after
pipe_wait()") this fixes a case where the pipe rewrite ended up caching
the pipe state incorrectly over a pipe lock drop event.

It wasn't quite as obvious, because you needed to splice data from a
pipe to a file, which is a fairly unusual operation, but it's completely
wrong.

Make sure we load the pipe head/tail/size information only after we've
waited for there to be data in the pipe.

While in that file, also make one of the splice helper functions use the
canonical arghument order for pipe_empty(). That's syntactic - pipe
emptiness is just that head and tail are equal, and thus mixing up head
and tail doesn't really matter. It's still wrong, though.

Reported-by: David Sterba <[e-mail]>
Cc: David Howells <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/splice.c

···
 	unsigned int mask = pipe->ring_size - 1;
 	int ret;
 
-	while (!pipe_empty(tail, head)) {
+	while (!pipe_empty(head, tail)) {
 		struct pipe_buffer *buf = &pipe->bufs[tail & mask];
 
 		sd->len = buf->len;
···
 	splice_from_pipe_begin(&sd);
 	while (sd.total_len) {
 		struct iov_iter from;
-		unsigned int head = pipe->head;
-		unsigned int tail = pipe->tail;
-		unsigned int mask = pipe->ring_size - 1;
+		unsigned int head, tail, mask;
 		size_t left;
 		int n;
 
···
 				break;
 			}
 		}
+
+		head = pipe->head;
+		tail = pipe->tail;
+		mask = pipe->ring_size - 1;
 
 		/* build the vector */
 		left = sd.total_len;