ipv4: keep skb->dst around in presence of IP options

ipv4: keep skb->dst around in presence of IP options

Andrey Konovalov got crashes in __ip_options_echo() when a NULL skb->dst
is accessed.

ipv4_pktinfo_prepare() should not drop the dst if (evil) IP options
are present.

We could refine the test to the presence of ts_needtime or srr,
but IP options are not often used, so let's be conservative.

Thanks to syzkaller team for finding this bug.

Fixes: d826eb14ecef ("ipv4: PKTINFO doesnt need dst reference")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 34b2cef20f19c87999fff3da4071e66937db9644)

Orabug: 25543766
CVE: CVE-2017-5970
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>