netfilter: nf_conntrack_gre: more locking around keymap list

gre_keymap_list should be protected in all places.
(unless I'm misreading something)

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>