In case of seg6 in encap mode, seg6_do_srh_encap() calls set_tun_src()
in order to set the src addr of outer IPv6 header.
The net_device is required for set_tun_src(). However calling ip6_dst_idev()
on dst_entry in case of IPv4 traffic results on the following bug.
Using just dst->dev should fix this BUG.
Fixes: 8936ef7604c11 ipv6: sr: fix NULL pointer dereference when setting encap source address
Signed-off-by: Ahmed Abdelsalam<amsalam20@xxxxxxxxx>