net: ipv6: fix use after free of struct seg6_pernet_data

sdata->tun_src should be freed before sdata is freed
because sdata->tun_src is allocated after sdata allocation.
So, kfree(sdata) and kfree(rcu_dereference_raw(sdata->tun_src)) are
changed code order.

Fixes: f04ed7d2 ("net: ipv6: check return value of rhashtable_init")

Signed-off-by: MichelleJin <shjy180909@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>