bcache: fixup btree_cache_wait list damage

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

bcache: fixup btree_cache_wait list damage

We get a kernel crash about "list_add corruption. next->prev should be

prev (ffff9c801bc01210), but was ffff9c77b688237c.

(next=ffffae586d8afe68)."

crash> struct list_head 0xffff9c801bc01210

struct list_head {

next = 0xffffae586d8afe68,

prev = 0xffffae586d8afe68

}

crash> struct list_head 0xffff9c77b688237c

struct list_head {

next = 0x0,

prev = 0x0

}

crash> struct list_head 0xffffae586d8afe68

struct list_head struct: invalid kernel virtual address: ffffae586d8afe68 type: "gdb_readmem_callback"

Cannot access memory at address 0xffffae586d8afe68

[230469.019492] Call Trace:

[230469.032041] prepare_to_wait+0x8a/0xb0

[230469.044363] ? bch_btree_keys_free+0x6c/0xc0 [escache]

[230469.056533] mca_cannibalize_lock+0x72/0x90 [escache]

[230469.068788] mca_alloc+0x2ae/0x450 [escache]

[230469.080790] bch_btree_node_get+0x136/0x2d0 [escache]

[230469.092681] bch_btree_check_thread+0x1e1/0x260 [escache]

[230469.104382] ? finish_wait+0x80/0x80

[230469.115884] ? bch_btree_check_recurse+0x1a0/0x1a0 [escache]

[230469.127259] kthread+0x112/0x130

[230469.138448] ? kthread_flush_work_fn+0x10/0x10

[230469.149477] ret_from_fork+0x35/0x40

bch_btree_check_thread() and bch_dirty_init_thread() may call

mca_cannibalize() to cannibalize other cached btree nodes. Only one thread

can do it at a time, so the op of other threads will be added to the

btree_cache_wait list.

We must call finish_wait() to remove op from btree_cache_wait before free

it's memory address. Otherwise, the list will be damaged. Also should call

bch_cannibalize_unlock() to release the btree_cache_alloc_lock and wake_up

other waiters.