unix/stream: fix peeking with an offset larger than data in queue

Currently, peeking on a unix stream socket with an offset larger than len of
the data in the sk receive queue returns immediately with bogus data.

This patch fixes this so that the behavior is the same as peeking with no
offset on an empty queue: the caller blocks.

Signed-off-by: Benjamin Poirier <bpoirier@suse.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>