splice: lift pipe_lock out of splice_to_pipe()

* splice_to_pipe() stops at pipe overflow and does *not* take pipe_lock
* ->splice_read() instances do the same
* vmsplice_to_pipe() and do_splice() (ultimate callers of splice_to_pipe())
  arrange for waiting, looping, etc. themselves.

That should make pipe_lock the outermost one.

Unfortunately, existing rules for the amount passed by vmsplice_to_pipe()
and do_splice() are quite ugly _and_ userland code can be easily broken
by changing those.  It's not even "no more than the maximal capacity of
this pipe" - it's "once we'd fed pipe->nr_buffers pages into the pipe,
leave instead of waiting".

Considering how poorly these rules are documented, let's try "wait for some
space to appear, unless given SPLICE_F_NONBLOCK, then push into pipe
and if we run into overflow, we are done".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fuse/dev.c

@@ -1364,7 +1364,6 @@ static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
 		goto out;
 
 	ret = 0;
-	pipe_lock(pipe);
 
 	if (!pipe->readers) {
 		send_sig(SIGPIPE, current, 0);
@@ -1400,7 +1399,6 @@ static ssize_t fuse_dev_splice_read(struct file *in, loff_t *ppos,
 	}
 
 out_unlock:
-	pipe_unlock(pipe);
 
 	if (do_wakeup) {
 		smp_mb();