xfrm: state: remove per-netns gc task

After commit 5b8ef341
("xfrm: Remove ancient sleeping when the SA is in acquire state")
gc does not need any per-netns data anymore.

As far as gc is concerned all state structs are the same, so we
can use a global work struct for it.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>