ipv4: ip_tunnel: use net namespace from rtable not socket

The socket parameter might legally be NULL, thus sock_net is sometimes
causing a NULL pointer dereference. Using net_device pointer in dst_entry
is more reliable.

Fixes: b6a7719a ("ipv4: hash net ptr into fragmentation bucket selection")
Reported-by: Rick Jones <rick.jones2@hp.com>
Cc: Rick Jones <rick.jones2@hp.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>