netfilter: nf_conntrack: add nf_ct_kill()

Encapsulate the common

	if (del_timer(&ct->timeout))
		ct->timeout.function((unsigned long)ct)

sequence in a new function.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>