net: fix __skb_try_recv_from_queue to return the old behavior

This function has to return NULL on a error case, because there is a
separate error variable.

The offset has to be changed only if skb is returned

v2: fix udp code to not use an extra variable

Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: David S. Miller <davem@davemloft.net>
Fixes: 65101aec ("net/sock: factor out dequeue/peek with offset cod")
Signed-off-by: Andrei Vagin <avagin@openvz.org>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>