sock: convert sk_peek_offset functions to WRITE_ONCE

Make the peek offset interface safe to use in lockless environments.
Use READ_ONCE and WRITE_ONCE to avoid race conditions between testing
and updating the peek offset.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>