netns PF_KEY: part 2

* interaction with userspace -- take netns from userspace socket.
* in ->notify hook take netns either from SA or explicitly passed --
	we don't know if SA/SPD flush is coming.
* stub policy migration with init_net for now.

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