ip6: fix skb leak in ip6frag_expire_frag_queue()

ip6: fix skb leak in ip6frag_expire_frag_queue()

Since ip6frag_expire_frag_queue() now pulls the head skb
from frag queue, we should no longer use skb_get(), since
this leads to an skb leak.

Stefan Bader initially reported a problem in 4.4.stable [1] caused
by the skb_get(), so this patch should also fix this issue.

296583.091021] kernel BUG at /build/linux-6VmqmP/linux-4.4.0/net/core/skbuff.c:1207!
[296583.091734] Call Trace:
[296583.091749]  [<ffffffff81740e50>] __pskb_pull_tail+0x50/0x350
[296583.091764]  [<ffffffff8183939a>] _decode_session6+0x26a/0x400
[296583.091779]  [<ffffffff817ec719>] __xfrm_decode_session+0x39/0x50
[296583.091795]  [<ffffffff818239d0>] icmpv6_route_lookup+0xf0/0x1c0
[296583.091809]  [<ffffffff81824421>] icmp6_send+0x5e1/0x940
[296583.091823]  [<ffffffff81753238>] ? __netif_receive_skb+0x18/0x60
[296583.091838]  [<ffffffff817532b2>] ? netif_receive_skb_internal+0x32/0xa0
[296583.091858]  [<ffffffffc0199f74>] ? ixgbe_clean_rx_irq+0x594/0xac0 [ixgbe]
[296583.091876]  [<ffffffffc04eb260>] ? nf_ct_net_exit+0x50/0x50 [nf_defrag_ipv6]
[296583.091893]  [<ffffffff8183d431>] icmpv6_send+0x21/0x30
[296583.091906]  [<ffffffff8182b500>] ip6_expire_frag_queue+0xe0/0x120
[296583.091921]  [<ffffffffc04eb27f>] nf_ct_frag6_expire+0x1f/0x30 [nf_defrag_ipv6]
[296583.091938]  [<ffffffff810f3b57>] call_timer_fn+0x37/0x140
[296583.091951]  [<ffffffffc04eb260>] ? nf_ct_net_exit+0x50/0x50 [nf_defrag_ipv6]
[296583.091968]  [<ffffffff810f5464>] run_timer_softirq+0x234/0x330
[296583.091982]  [<ffffffff8108a339>] __do_softirq+0x109/0x2b0

Fixes: d4289fcc9b16 ("net: IP6 defrag: use rbtrees for IPv6 defrag")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Stefan Bader <stefan.bader@canonical.com>
Cc: Peter Oskolkov <posk@google.com>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>