[ICMP]: Store sock rather than socket for ICMP flow control.

Basically, there is no difference, what to store: socket or sock. Though,
sock looks better as there will be 1 less dereferrence on the fast path.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>