ipv4: Initialize on-stack cork more efficiently.

ip_setup_cork() explicitly initializes every member of
inet_cork except flags, addr, and opt.  So we can simply
set those three members to zero instead of using a
memset() via an empty struct assignment.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>