btrfs: fix BUG_ON condition in btrfs_cancel_balance

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

btrfs: fix BUG_ON condition in btrfs_cancel_balance

Pausing and canceling balance can race to interrupt balance lead to BUG_ON

panic in btrfs_cancel_balance. The BUG_ON condition in btrfs_cancel_balance

does not take this race scenario into account.

However, the race condition has no other side effects. We can fix that.

Reproducing it with panic trace like this:

kernel BUG at fs/btrfs/volumes.c:4618!

RIP: 0010:btrfs_cancel_balance+0x5cf/0x6a0

Call Trace:

<TASK>

? do_nanosleep+0x60/0x120

? hrtimer_nanosleep+0xb7/0x1a0

? sched_core_clone_cookie+0x70/0x70

btrfs_ioctl_balance_ctl+0x55/0x70

btrfs_ioctl+0xa46/0xd20

__x64_sys_ioctl+0x7d/0xa0

do_syscall_64+0x38/0x80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

Race scenario as follows:

> mutex_unlock(&fs_info->balance_mutex);

> --------------------

> .......issue pause and cancel req in another thread

> --------------------

> ret = __btrfs_balance(fs_info);

>

> mutex_lock(&fs_info->balance_mutex);

> if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req)) {

> btrfs_info(fs_info, "balance: paused");

> btrfs_exclop_balance(fs_info, BTRFS_EXCLOP_BALANCE_PAUSED);

> }