net/unix: fix logic about sk_peek_offset

Now send with MSG_PEEK can return data from multiple SKBs.

Unfortunately we take into account the peek offset for each skb,
that is wrong. We need to apply the peek offset only once.

In addition, the peek offset should be used only if MSG_PEEK is set.

Cc: "David S. Miller" <davem@davemloft.net> (maintainer:NETWORKING
Cc: Eric Dumazet <edumazet@google.com> (commit_signer:1/14=7%)
Cc: Aaron Conole <aconole@bytheb.org>
Fixes: 9f389e35 ("af_unix: return data from multiple SKBs on recv() with MSG_PEEK flag")
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Tested-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: David S. Miller <davem@davemloft.net>