gtp: fix use-after-free and null-ptr-deref in gtp_newlink()

In the Linux kernel, the following vulnerability has been resolved:

gtp: fix use-after-free and null-ptr-deref in gtp_newlink()

The gtp_link_ops operations structure for the subsystem must be
registered after registering the gtp_net_ops pernet operations structure.

Syzkaller hit 'general protection fault in gtp_genl_dump_pdp' bug:

[ 1010.702740] gtp: GTP module unloaded
[ 1010.715877] general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] SMP KASAN NOPTI
[ 1010.715888] KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
[ 1010.715895] CPU: 1 PID: 128616 Comm: a.out Not tainted 6.8.0-rc6-std-def-alt1 #1
[ 1010.715899] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.0-alt1 04/01/2014
[ 1010.715908] RIP: 0010:gtp_newlink+0x4d7/0x9c0 [gtp]
[ 1010.715915] Code: 80 3c 02 00 0f 85 41 04 00 00 48 8b bb d8 05 00 00 e8 ed f6 ff ff 48 89 c2 48 89 c5 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f 85 4f 04 00 00 4c 89 e2 4c 8b 6d 00 48 b8 00 00 00
[ 1010.715920] RSP: 0018:ffff888020fbf180 EFLAGS: 00010203
[ 1010.715929] RAX: dffffc0000000000 RBX: ffff88800399c000 RCX: 0000000000000000
[ 1010.715933] RDX: 0000000000000001 RSI: ffffffff84805280 RDI: 0000000000000282
[ 1010.715938] RBP: 000000000000000d R08: 0000000000000001 R09: 0000000000000000
[ 1010.715942] R10: 0000000000000001 R11: 0000000000000001 R12: ffff88800399cc80
[ 1010.715947] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000400
[ 1010.715953] FS: 00007fd1509ab5c0(0000) GS:ffff88805b300000(0000) knlGS:0000000000000000
[ 1010.715958] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1010.715962] CR2: 0000000000000000 CR3: 000000001c07a000 CR4: 0000000000750ee0
[ 1010.715968] PKRU: 55555554
[ 1010.715972] Call Trace:
[ 1010.715985] ? __die_body.cold+0x1a/0x1f
[ 1010.715995] ? die_addr+0x43/0x70
[ 1010.716002] ? exc_general_protection+0x199/0x2f0
[ 1010.716016] ? asm_exc_general_protection+0x1e/0x30
[ 1010.716026] ? gtp_newlink+0x4d7/0x9c0 [gtp]
[ 1010.716034] ? gtp_net_exit+0x150/0x150 [gtp]
[ 1010.716042] __rtnl_newlink+0x1063/0x1700
[ 1010.716051] ? rtnl_setlink+0x3c0/0x3c0
[ 1010.716063] ? is_bpf_text_address+0xc0/0x1f0
[ 1010.716070] ? kernel_text_address.part.0+0xbb/0xd0
[ 1010.716076] ? __kernel_text_address+0x56/0xa0
[ 1010.716084] ? unwind_get_return_address+0x5a/0xa0
[ 1010.716091] ? create_prof_cpu_mask+0x30/0x30
[ 1010.716098] ? arch_stack_walk+0x9e/0xf0
[ 1010.716106] ? stack_trace_save+0x91/0xd0
[ 1010.716113] ? stack_trace_consume_entry+0x170/0x170
[ 1010.716121] ? __lock_acquire+0x15c5/0x5380
[ 1010.716139] ? mark_held_locks+0x9e/0xe0
[ 1010.716148] ? kmem_cache_alloc_trace+0x35f/0x3c0
[ 1010.716155] ? __rtnl_newlink+0x1700/0x1700
[ 1010.716160] rtnl_newlink+0x69/0xa0
[ 1010.716166] rtnetlink_rcv_msg+0x43b/0xc50
[ 1010.716172] ? rtnl_fdb_dump+0x9f0/0x9f0
[ 1010.716179] ? lock_acquire+0x1fe/0x560
[ 1010.716188] ? netlink_deliver_tap+0x12f/0xd50
[ 1010.716196] netlink_rcv_skb+0x14d/0x440
[ 1010.716202] ? rtnl_fdb_dump+0x9f0/0x9f0
[ 1010.716208] ? netlink_ack+0xab0/0xab0
[ 1010.716213] ? netlink_deliver_tap+0x202/0xd50
[ 1010.716220] ? netlink_deliver_tap+0x218/0xd50
[ 1010.716226] ? __virt_addr_valid+0x30b/0x590
[ 1010.716233] netlink_unicast+0x54b/0x800
[ 1010.716240] ? netlink_attachskb+0x870/0x870
[ 1010.716248] ? __check_object_size+0x2de/0x3b0
[ 1010.716254] netlink_sendmsg+0x938/0xe40
[ 1010.716261] ? netlink_unicast+0x800/0x800
[ 1010.716269] ? __import_iovec+0x292/0x510
[ 1010.716276] ? netlink_unicast+0x800/0x800
[ 1010.716284] __sock_sendmsg+0x159/0x190
[ 1010.716290] ____sys_sendmsg+0x712/0x880
[ 1010.716297] ? sock_write_iter+0x3d0/0x3d0
[ 1010.716304] ? __ia32_sys_recvmmsg+0x270/0x270
[ 1010.716309] ? lock_acquire+0x1fe/0x560
[ 1010.716315] ? drain_array_locked+0x90/0x90
[ 1010.716324] ___sys_sendmsg+0xf8/0x170
[ 1010.716331] ? sendmsg_copy_msghdr+0x170/0x170
[ 1010.716337] ? lockdep_init_map
---truncated---