Commit e2d118a1 ("net: inet: Support UID-based routing in IP
protocols.") made __build_flow_key call sock_net(sk) to determine
the network namespace of the passed-in socket. This crashes if sk
is NULL.

Fix this by getting the network namespace from the skb instead.

Fixes: e2d118a1 ("net: inet: Support UID-based routing in IP protocols.")
Reported-by: NErez Shitrit <erezsh@dev.mellanox.co.il>
Signed-off-by: NLorenzo Colitti <lorenzo@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>