tcp: fix MSG_SENDPAGE_NOTLAST logic

commit 35f9c09f (tcp: tcp_sendpages() should call tcp_push() once)
added an internal flag : MSG_SENDPAGE_NOTLAST meant to be set on all
frags but the last one for a splice() call.

The condition used to set the flag in pipe_to_sendpage() relied on
splice() user passing the exact number of bytes present in the pipe,
or a smaller one.

But some programs pass an arbitrary high value, and the test fails.

The effect of this bug is a lack of tcp_push() at the end of a
splice(pipe -> socket) call, and possibly very slow or erratic TCP
sessions.

We should both test sd->total_len and fact that another fragment
is in the pipe (pipe->nrbufs > 1)

Many thanks to Willy for providing very clear bug report, bisection
and test programs.
Reported-by: NWilly Tarreau <w@1wt.eu>
Bisected-by: NWilly Tarreau <w@1wt.eu>
Tested-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

fs/splice.c

@@ -696,8 +696,10 @@ static int pipe_to_sendpage(struct pipe_inode_info *pipe,
 		return -EINVAL;
 
 	more = (sd->flags & SPLICE_F_MORE) ? MSG_MORE : 0;
-	if (sd->len < sd->total_len)
+
+	if (sd->len < sd->total_len && pipe->nrbufs > 1)
 		more |= MSG_SENDPAGE_NOTLAST;
+
 	return file->f_op->sendpage(file, buf->page, buf->offset,
 				    sd->len, &pos, more);
 }