netns xfrm: fix "ip xfrm state|policy count" misreport

"ip xfrm state|policy count" report SA/SP count from init_net,
not from netns of caller process.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>