splice: reinstate SIGPIPE/EPIPE handling

Commit 8924feff66f3 ("splice: lift pipe_lock out of splice_to_pipe()")
caused a regression when there were no more readers left on a pipe that
was being spliced into: rather than the expected SIGPIPE and -EPIPE
return value, the writer would end up waiting forever for space to free
up (which obviously was not going to happen with no readers around).

Fixes: 8924feff66f3 ("splice: lift pipe_lock out of splice_to_pipe()")
Reported-and-tested-by: Andreas Schwab <[e-mail]>
Debugged-by: Al Viro <[e-mail]>
Cc: [e-mail] # v4.9
Signed-off-by: Linus Torvalds <[e-mail]>

fs/splice.c

···
 
 static int wait_for_space(struct pipe_inode_info *pipe, unsigned flags)
 {
-	while (pipe->nrbufs == pipe->buffers) {
+	for (;;) {
+		if (unlikely(!pipe->readers)) {
+			send_sig(SIGPIPE, current, 0);
+			return -EPIPE;
+		}
+		if (pipe->nrbufs != pipe->buffers)
+			return 0;
 		if (flags & SPLICE_F_NONBLOCK)
 			return -EAGAIN;
 		if (signal_pending(current))
···
 		pipe_wait(pipe);
 		pipe->waiting_writers--;
 	}
-	return 0;
 }
 
 static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,