btrfs: fix hang during unmount when block group reclaim task is running

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

btrfs: fix hang during unmount when block group reclaim task is running

When we start an unmount, at close_ctree(), if we have the reclaim task
running and in the middle of a data block group relocation, we can trigger
a deadlock when stopping an async reclaim task, producing a trace like the
following:

[629724.498185] task:kworker/u16:7 state:D stack: 0 pid:681170 ppid: 2 flags:0x00004000
[629724.499760] Workqueue: events_unbound btrfs_async_reclaim_metadata_space [btrfs]
[629724.501267] Call Trace:
[629724.501759] <TASK>
[629724.502174] __schedule+0x3cb/0xed0
[629724.502842] schedule+0x4e/0xb0
[629724.503447] btrfs_wait_on_delayed_iputs+0x7c/0xc0 [btrfs]
[629724.504534] ? prepare_to_wait_exclusive+0xc0/0xc0
[629724.505442] flush_space+0x423/0x630 [btrfs]
[629724.506296] ? rcu_read_unlock_trace_special+0x20/0x50
[629724.507259] ? lock_release+0x220/0x4a0
[629724.507932] ? btrfs_get_alloc_profile+0xb3/0x290 [btrfs]
[629724.508940] ? do_raw_spin_unlock+0x4b/0xa0
[629724.509688] btrfs_async_reclaim_metadata_space+0x139/0x320 [btrfs]
[629724.510922] process_one_work+0x252/0x5a0
[629724.511694] ? process_one_work+0x5a0/0x5a0
[629724.512508] worker_thread+0x52/0x3b0
[629724.513220] ? process_one_work+0x5a0/0x5a0
[629724.514021] kthread+0xf2/0x120
[629724.514627] ? kthread_complete_and_exit+0x20/0x20
[629724.515526] ret_from_fork+0x22/0x30
[629724.516236] </TASK>
[629724.516694] task:umount state:D stack: 0 pid:719055 ppid:695412 flags:0x00004000
[629724.518269] Call Trace:
[629724.518746] <TASK>
[629724.519160] __schedule+0x3cb/0xed0
[629724.519835] schedule+0x4e/0xb0
[629724.520467] schedule_timeout+0xed/0x130
[629724.521221] ? lock_release+0x220/0x4a0
[629724.521946] ? lock_acquired+0x19c/0x420
[629724.522662] ? trace_hardirqs_on+0x1b/0xe0
[629724.523411] __wait_for_common+0xaf/0x1f0
[629724.524189] ? usleep_range_state+0xb0/0xb0
[629724.524997] __flush_work+0x26d/0x530
[629724.525698] ? flush_workqueue_prep_pwqs+0x140/0x140
[629724.526580] ? lock_acquire+0x1a0/0x310
[629724.527324] __cancel_work_timer+0x137/0x1c0
[629724.528190] close_ctree+0xfd/0x531 [btrfs]
[629724.529000] ? evict_inodes+0x166/0x1c0
[629724.529510] generic_shutdown_super+0x74/0x120
[629724.530103] kill_anon_super+0x14/0x30
[629724.530611] btrfs_kill_super+0x12/0x20 [btrfs]
[629724.531246] deactivate_locked_super+0x31/0xa0
[629724.531817] cleanup_mnt+0x147/0x1c0
[629724.532319] task_work_run+0x5c/0xa0
[629724.532984] exit_to_user_mode_prepare+0x1a6/0x1b0
[629724.533598] syscall_exit_to_user_mode+0x16/0x40
[629724.534200] do_syscall_64+0x48/0x90
[629724.534667] entry_SYSCALL_64_after_hwframe+0x44/0xae
[629724.535318] RIP: 0033:0x7fa2b90437a7
[629724.535804] RSP: 002b:00007ffe0b7e4458 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
[629724.536912] RAX: 0000000000000000 RBX: 00007fa2b9182264 RCX: 00007fa2b90437a7
[629724.538156] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000555d6cf20dd0
[629724.539053] RBP: 0000555d6cf20ba0 R08: 0000000000000000 R09: 00007ffe0b7e3200
[629724.539956] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[629724.540883] R13: 0000555d6cf20dd0 R14: 0000555d6cf20cb0 R15: 0000000000000000
[629724.541796] </TASK>

This happens because:

1) Before entering close_ctree() we have the async block group reclaim
task running and relocating a data block group;

2) There's an async metadata (or data) space reclaim task running;

3) We enter close_ctree() and park the cleaner kthread;

4) The async space reclaim task is at flush_space() and runs all the
existing delayed iputs;

5) Before the async space reclaim task calls
btrfs_wait_on_delayed_iputs(), the block group reclaim task which is
doing the data block group relocation, creates a delayed iput at
replace_file_extents() (called when COWing leaves that have file extent
items pointing to relocated data exten
---truncated---