netfilter: netns nf_conntrack: GRE conntracking in netns

* make keymap list per-netns
* per-netns keymal lock (not strictly necessary)
* flush keymap at netns stop and module unload.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>