ipv4_pdp_find() is called in TX packet path of GTP.
ipv4_pdp_find() internally uses gtp->tid_hash to lookup pdp context.
In the current code, gtp->tid_hash and gtp->addr_hash are freed by
->dellink(), which is gtp_dellink().
But gtp_dellink() would be called while packets are processing.
So, gtp_dellink() should not free gtp->tid_hash and gtp->addr_hash.
Instead, dev->priv_destructor() would be used because this callback
is called after all packet processing safely.

Test commands:
    ip link add veth1 type veth peer name veth2
    ip a a 172.0.0.1/24 dev veth1
    ip link set veth1 up
    ip a a 172.99.0.1/32 dev lo

    gtp-link add gtp1 &

    gtp-tunnel add gtp1 v1 200 100 172.99.0.2 172.0.0.2
    ip r a  172.99.0.2/32 dev gtp1
    ip link set gtp1 mtu 1500

    ip netns add ns2
    ip link set veth2 netns ns2
    ip netns exec ns2 ip a a 172.0.0.2/24 dev veth2
    ip netns exec ns2 ip link set veth2 up
    ip netns exec ns2 ip a a 172.99.0.2/32 dev lo
    ip netns exec ns2 ip link set lo up

    ip netns exec ns2 gtp-link add gtp2 &
    ip netns exec ns2 gtp-tunnel add gtp2 v1 100 200 172.99.0.1 172.0.0.1
    ip netns exec ns2 ip r a 172.99.0.1/32 dev gtp2
    ip netns exec ns2 ip link set gtp2 mtu 1500

    hping3 172.99.0.2 -2 --flood &
    ip link del gtp1

Splat looks like:
[   72.568081][ T1195] BUG: KASAN: use-after-free in ipv4_pdp_find.isra.12+0x130/0x170 [gtp]
[   72.568916][ T1195] Read of size 8 at addr ffff8880b9a35d28 by task hping3/1195
[   72.569631][ T1195]
[   72.569861][ T1195] CPU: 2 PID: 1195 Comm: hping3 Not tainted 5.5.0-rc1 #199
[   72.570547][ T1195] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[   72.571438][ T1195] Call Trace:
[   72.571764][ T1195]  dump_stack+0x96/0xdb
[   72.572171][ T1195]  ? ipv4_pdp_find.isra.12+0x130/0x170 [gtp]
[   72.572761][ T1195]  print_address_description.constprop.5+0x1be/0x360
[   72.573400][ T1195]  ? ipv4_pdp_find.isra.12+0x130/0x170 [gtp]
[   72.573971][ T1195]  ? ipv4_pdp_find.isra.12+0x130/0x170 [gtp]
[   72.574544][ T1195]  __kasan_report+0x12a/0x16f
[   72.575014][ T1195]  ? ipv4_pdp_find.isra.12+0x130/0x170 [gtp]
[   72.575593][ T1195]  kasan_report+0xe/0x20
[   72.576004][ T1195]  ipv4_pdp_find.isra.12+0x130/0x170 [gtp]
[   72.576577][ T1195]  gtp_build_skb_ip4+0x199/0x1420 [gtp]
[ ... ]
[   72.647671][ T1195] BUG: unable to handle page fault for address: ffff8880b9a35d28
[   72.648512][ T1195] #PF: supervisor read access in kernel mode
[   72.649158][ T1195] #PF: error_code(0x0000) - not-present page
[   72.649849][ T1195] PGD a6c01067 P4D a6c01067 PUD 11fb07067 PMD 11f939067 PTE 800fffff465ca060
[   72.652958][ T1195] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[   72.653834][ T1195] CPU: 2 PID: 1195 Comm: hping3 Tainted: G    B             5.5.0-rc1 #199
[   72.668062][ T1195] RIP: 0010:ipv4_pdp_find.isra.12+0x86/0x170 [gtp]
[ ... ]
[   72.679168][ T1195] Call Trace:
[   72.679603][ T1195]  gtp_build_skb_ip4+0x199/0x1420 [gtp]
[   72.681915][ T1195]  ? ipv4_pdp_find.isra.12+0x170/0x170 [gtp]
[   72.682513][ T1195]  ? lock_acquire+0x164/0x3b0
[   72.682966][ T1195]  ? gtp_dev_xmit+0x35e/0x890 [gtp]
[   72.683481][ T1195]  gtp_dev_xmit+0x3c2/0x890 [gtp]
[ ... ]

Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>