syzkaller got crashes at dismantle time [1]

It is not correct to test (tun->flags & IFF_NAPI) in tun_napi_disable()
and tun_napi_del() : Each tun_file can have different mode, depending
on how they were created.

Similarly I have changed tun_get_user() and tun_poll_controller()
to use the new tfile->napi_enabled boolean.

[  154.331360] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  154.339220] IP: [<ffffffff9634cad6>] hrtimer_active+0x26/0x60
[  154.344983] PGD 0
[  154.347009] Oops: 0000 [#1] SMP
[  154.350680] gsmi: Log Shutdown Reason 0x03
[  154.379572] task: ffff994719150dc0 ti: ffff99475c0ae000 task.ti: ffff99475c0ae000
[  154.387043] RIP: 0010:[<ffffffff9634cad6>]  [<ffffffff9634cad6>] hrtimer_active+0x26/0x60
[  154.395232] RSP: 0018:ffff99475c0afce8  EFLAGS: 00010246
[  154.400542] RAX: ffff994754850ac0 RBX: ffff994753e65408 RCX: ffff994753e65388
[  154.407666] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff994753e65408
[  154.414790] RBP: ffff99475c0afce8 R08: 0000000000000000 R09: 0000000000000000
[  154.421921] R10: ffff99475f6f5910 R11: 0000000000000001 R12: 0000000000000000
[  154.429044] R13: ffff99417deab668 R14: ffff99417deaa780 R15: ffff99475f45dde0
[  154.436174] FS:  0000000000000000(0000) GS:ffff994767a00000(0000) knlGS:0000000000000000
[  154.444249] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  154.449986] CR2: 0000000000000000 CR3: 00000005a8a0e000 CR4: 0000000000022670
[  154.457110] Stack:
[  154.459120]  ffff99475c0afd28 ffffffff9634d614 1000000000000000 0000000000000000
[  154.466598]  ffffe54240000000 ffff994753e65408 ffff994753e653a8 ffff99417deab668
[  154.474067]  ffff99475c0afd48 ffffffff9634d6fd ffff99474c2be678 ffff994753e65398
[  154.481537] Call Trace:
[  154.483985]  [<ffffffff9634d614>] hrtimer_try_to_cancel+0x24/0xf0
[  154.490074]  [<ffffffff9634d6fd>] hrtimer_cancel+0x1d/0x30
[  154.495563]  [<ffffffff96860b3c>] napi_disable+0x3c/0x70
[  154.500875]  [<ffffffff9678ae62>] __tun_detach+0xd2/0x360
[  154.506272]  [<ffffffff9678b117>] tun_chr_close+0x27/0x40
[  154.511669]  [<ffffffff9646ebe6>] __fput+0xd6/0x1e0
[  154.516548]  [<ffffffff9646ed3e>] ____fput+0xe/0x10
[  154.521429]  [<ffffffff963035a2>] task_work_run+0x72/0x90
[  154.526827]  [<ffffffff962e9407>] do_exit+0x317/0xb60
[  154.531879]  [<ffffffff962e9c8f>] do_group_exit+0x3f/0xa0
[  154.537275]  [<ffffffff962e9d07>] SyS_exit_group+0x17/0x20
[  154.542769]  [<ffffffff969784be>] entry_SYSCALL_64_fastpath+0x12/0x17

Fixes: 94317099 ("net-tun: enable NAPI for TUN/TAP driver")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>