amt: use cancel_delayed_work() instead of flush_delayed_work() in amt_fini()

amt: use cancel_delayed_work() instead of flush_delayed_work() in amt_fini()

When the amt module is being removed, it calls flush_delayed_work() to exit
source_gc_wq. But it wouldn't be exited properly because the
amt_source_gc_work(), which is the callback function of source_gc_wq
internally calls mod_delayed_work() again.
So, amt_source_gc_work() would be called after the amt module is removed.
Therefore kernel panic would occur.
In order to avoid it, cancel_delayed_work() should be used instead of
flush_delayed_work().

Test commands:
   modprobe amt
   modprobe -rv amt

Splat looks like:
 BUG: unable to handle page fault for address: fffffbfff80f50db
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 1237ee067 P4D 1237ee067 PUD 1237b2067 PMD 100c11067 PTE 0
 Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN PTI
 CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0+ #27
 5a0ebebc29fe5c40c68bea90197606c3a832b09f
 RIP: 0010:run_timer_softirq+0x221/0xfc0
 Code: 00 00 4c 89 e1 4c 8b 30 48 c1 e9 03 80 3c 29 00 0f 85 ed 0b 00 00
 4d 89 34 24 4d 85 f6 74 19 49 8d 7e 08 48 89 f9 48 c1 e9 03 <80> 3c 29 00
 0f 85 fa 0b 00 00 4d 89 66 08 83 04 24 01 49 89 d4 48
 RSP: 0018:ffff888119009e50 EFLAGS: 00010806
 RAX: ffff8881191f8a80 RBX: 00000000007ffe2a RCX: 1ffffffff80f50db
 RDX: ffff888119009ed0 RSI: 0000000000000008 RDI: ffffffffc07a86d8
 RBP: dffffc0000000000 R08: ffff8881191f8280 R09: ffffed102323f061
 R10: ffff8881191f8307 R11: ffffed102323f060 R12: ffff888119009ec8
 R13: 00000000000000c0 R14: ffffffffc07a86d0 R15: ffff8881191f82e8
 FS:  0000000000000000(0000) GS:ffff888119000000(0000)
 knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: fffffbfff80f50db CR3: 00000001062dc002 CR4: 00000000003706e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  <IRQ>
  ? add_timer+0x650/0x650
  ? kvm_clock_read+0x14/0x30
  ? ktime_get+0xb9/0x180
  ? rcu_read_lock_held_common+0xe/0xa0
  ? rcu_read_lock_sched_held+0x56/0xc0
  ? rcu_read_lock_bh_held+0xa0/0xa0
  ? hrtimer_interrupt+0x271/0x790
  __do_softirq+0x1d0/0x88f
  irq_exit_rcu+0xe7/0x120
  sysvec_apic_timer_interrupt+0x8a/0xb0
  </IRQ>
  <TASK>
[ ... ]

Fixes: bc54e49c140b ("amt: add multicast(IGMP) report message handler")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20211108145340.17208-1-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>