xfrm: Fix NULL pointer dereference on sub policy usage

xfrm_state_sort() takes the unsorted states from the src array
and stores them into the dst array. We try to get the namespace
from the dst array which is empty at this time, so take the
namespace from the src array instead.

Fixes: 283bc9f3 ("xfrm: Namespacify xfrm state/policy locks")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>