ipsec: fix IPv6 over IPv4 tunneling.

Properly initialize setdf variable in ipsec_encap().
It is used for AF_INET6 case when IPv6 datagram is going to be
encapsulated into IPv4 datagram.

PR: 282535
Fixes: 4046178557e1

(cherry picked from commit c94d6389e428fac55946bfcdbbc3162c06a9278e)