tcp: Fix double sizeof in new tcp_metrics code

Fix double sizeof when parsing IPv6 address from
user space because it breaks get/del by specific IPv6 address.

	Problem noticed by David Binderman:

https://bugzilla.kernel.org/show_bug.cgi?id=49171

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>