netfilter: nf_conntrack_gre: nf_ct_gre_keymap_flush() fixlet

It does "kfree(list_head)" which looks wrong because entity that was
allocated is definitely not list_head.

However, this all works because list_head is first item in
struct nf_ct_gre_keymap.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>