ipv6: seg6_genl_set_tunsrc() must check kmemdup() return value

seg6_genl_get_tunsrc() and set_tun_src() do not handle tun_src being
possibly NULL, so we must check kmemdup() return value and abort if
it is NULL

Fixes: 915d7e5e ("ipv6: sr: add code base for control plane support of SR-IPv6")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Lebrun <david.lebrun@uclouvain.be>
Acked-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>