pipe: Allow buffers to be marked read-whole-or-error for notifications

Allow a buffer to be marked such that read() must return the entire buffer
in one go or return ENOBUFS.  Multiple buffers can be amalgamated into a
single read, but a short read will occur if the next "whole" buffer won't
fit.

This is useful for watch queue notifications to make sure we don't split a
notification across multiple reads, especially given that we need to
fabricate an overrun record under some circumstances - and that isn't in
the buffers.

Signed-off-by: David Howells <dhowells@redhat.com>

kernel/watch_queue.c

@@ -115,7 +115,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
 	buf->ops = &watch_queue_pipe_buf_ops;
 	buf->offset = offset;
 	buf->len = len;
-	buf->flags = 0;
+	buf->flags = PIPE_BUF_FLAG_WHOLE;
 	pipe->head = head + 1;
 
 	if (!test_and_clear_bit(note, wqueue->notes_bitmap)) {