sit: check if IPv6 enabled before calling ip6_err_gen_icmpv6_unreach()

If we disabled IPv6 from the kernel command line (ipv6.disable=1), we should
not call ip6_err_gen_icmpv6_unreach(). This:

  ip link add sit1 type sit local 192.0.2.1 remote 192.0.2.2 ttl 1
  ip link set sit1 up
  ip addr add 198.51.100.1/24 dev sit1
  ping 198.51.100.2

if IPv6 is disabled at boot time, will crash the kernel.

v2: there's no need to use in6_dev_get(), use __in6_dev_get() instead,
    as we only need to check that idev exists and we are under
    rcu_read_lock() (from netif_receive_skb_internal()).

Reported-by: Jianlin Shi <jishi@redhat.com>
Fixes: ca15a078 ("sit: generate icmpv6 error when receiving icmpv4 error")
Cc: Oussama Ghorbel <ghorbel@pivasoftware.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>