UPSTREAM: net: ipv4: Don't crash if passing a null sk to ip_do_redirect.

Commit e2d118a1cb5e ("net: inet: Support UID-based routing in IP
protocols.") made ip_do_redirect 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: e2d118a1cb5e ("net: inet: Support UID-based routing in IP protocols.")
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Fixes: android-4.9 commitID 5044292c
       ("UPSTREAM: net: inet: Support UID-based routing in IP protocols.")
(cherry picked from commit 7d99569460eae28b187d574aec930a4cf8b90441)
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>