net: Make flow cache namespace-aware

flow_cache_lookup will return a cached object (or null pointer) that the
resolver (i.e. xfrm_policy_lookup) previously found for another namespace
using the same key/family/dir.  Instead, make the namespace part of what
identifies entries in the cache.

As before, flow_entry_valid will return 0 for entries where the namespace
has been deleted, and they will be removed from the cache the next time
flow_cache_gc_task is run.

Reported-by: Andrew Dickinson <whydna@whydna.net>
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>