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

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>
Change-Id: I16a3c343cb142c482ca6dd363c28b3a12d73a46d
Fixes: Change-Id: I910504b508948057912bc188fd1e8aca28294de3
       ("net: inet: Support UID-based routing in IP protocols.")
(cherry picked from commit 7d99569460eae28b187d574aec930a4cf8b90441)
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>