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

Commit e2d118a1cb5e ("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.

[Backport of net-next d109e61bfe7a468fd8df4a7ceb65635e7aa909a0]

Bug: 16355602
Change-Id: I27161b70f448bb95adce3994a97920d54987ce4e
Fixes: e2d118a1cb5e ("net: inet: Support UID-based routing in IP protocols.")
Reported-by: Erez Shitrit <erezsh@dev.mellanox.co.il>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>