sock: correctly test SOCK_TIMESTAMP in sock_recv_ts_and_drops()

It seems the code does not match the intent.

This broke packetdrill, and probably other programs.

Fixes: 6c7c98ba

 ("sock: avoid dirtying sk_stamp, if possible")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>