The access to non-existent netlink attribute causes to the following
kernel panic. Fix it by checking existence before trying to read it.

  general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  CPU: 0 PID: 6744 Comm: syz-executor.0 Not tainted 5.15.0-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  RIP: 0010:nla_get_u32 include/net/netlink.h:1554 [inline]
  RIP: 0010:nldev_stat_set_mode_doit drivers/infiniband/core/nldev.c:1909 [inline]
  RIP: 0010:nldev_stat_set_doit+0x578/0x10d0 drivers/infiniband/core/nldev.c:2040
  Code: fa 4c 8b a4 24 f8 02 00 00 48 b8 00 00 00 00 00 fc ff df c7 84 24 80 00 00 00 00 00 00 00 49 8d 7c 24 04 48 89
  fa 48 c1 ea 03 <0f> b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 02
  RSP: 0018:ffffc90004acf2e8 EFLAGS: 00010247
  RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffc90002b94000
  RDX: 0000000000000000 RSI: ffffffff8684c5ff RDI: 0000000000000004
  RBP: ffff88807cda4000 R08: 0000000000000000 R09: ffff888023fb8027
  R10: ffffffff8684c5d7 R11: 0000000000000000 R12: 0000000000000000
  R13: 0000000000000001 R14: ffff888041024280 R15: ffff888031ade780
  FS:  00007eff9dddd700(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000001b2ef24000 CR3: 0000000036902000 CR4: 00000000003506f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   <TASK>
   rdma_nl_rcv_msg+0x36d/0x690 drivers/infiniband/core/netlink.c:195
   rdma_nl_rcv_skb drivers/infiniband/core/netlink.c:239 [inline]
   rdma_nl_rcv+0x2ee/0x430 drivers/infiniband/core/netlink.c:259
   netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
   netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1345
   netlink_sendmsg+0x86d/0xda0 net/netlink/af_netlink.c:1916
   sock_sendmsg_nosec net/socket.c:704 [inline]
   sock_sendmsg+0xcf/0x120 net/socket.c:724
   ____sys_sendmsg+0x6e8/0x810 net/socket.c:2409
   ___sys_sendmsg+0xf3/0x170 net/socket.c:2463
   __sys_sendmsg+0xe5/0x1b0 net/socket.c:2492
   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
   do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
   entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 822cf785 ("RDMA/nldev: Split nldev_stat_set_mode_doit out of nldev_stat_set_doit")
Link: https://lore.kernel.org/r/b21967c366f076ff1988862f9c8a1aa0244c599f.1637151999.git.leonro@nvidia.com
Reported-by: syzbot+9111d2255a9710e87562@syzkaller.appspotmail.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>