netns xfrm: per-netns state GC list

km_waitq is going to be made per-netns to disallow spurious wakeups
in __xfrm_lookup().

To not wakeup after every garbage-collected xfrm_state (which potentially
can be from different netns) make state GC list per-netns.

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