dm cache: free background tracker's queued work in btracker_destroy

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

dm cache: free background tracker's queued work in btracker_destroy

Otherwise the kernel can BUG with:

[ 2245.426978] =============================================================================
[ 2245.435155] BUG bt_work (Tainted: G B W ): Objects remaining in bt_work on __kmem_cache_shutdown()
[ 2245.445233] -----------------------------------------------------------------------------
[ 2245.445233]
[ 2245.454879] Slab 0x00000000b0ce2b30 objects=64 used=2 fp=0x000000000a3c6a4e flags=0x17ffffc0000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
[ 2245.467300] CPU: 7 PID: 10805 Comm: lvm Kdump: loaded Tainted: G B W 6.0.0-rc2 #19
[ 2245.476078] Hardware name: Dell Inc. PowerEdge R7525/0590KW, BIOS 2.5.6 10/06/2021
[ 2245.483646] Call Trace:
[ 2245.486100] <TASK>
[ 2245.488206] dump_stack_lvl+0x34/0x48
[ 2245.491878] slab_err+0x95/0xcd
[ 2245.495028] __kmem_cache_shutdown.cold+0x31/0x136
[ 2245.499821] kmem_cache_destroy+0x49/0x130
[ 2245.503928] btracker_destroy+0x12/0x20 [dm_cache]
[ 2245.508728] smq_destroy+0x15/0x60 [dm_cache_smq]
[ 2245.513435] dm_cache_policy_destroy+0x12/0x20 [dm_cache]
[ 2245.518834] destroy+0xc0/0x110 [dm_cache]
[ 2245.522933] dm_table_destroy+0x5c/0x120 [dm_mod]
[ 2245.527649] __dm_destroy+0x10e/0x1c0 [dm_mod]
[ 2245.532102] dev_remove+0x117/0x190 [dm_mod]
[ 2245.536384] ctl_ioctl+0x1a2/0x290 [dm_mod]
[ 2245.540579] dm_ctl_ioctl+0xa/0x20 [dm_mod]
[ 2245.544773] __x64_sys_ioctl+0x8a/0xc0
[ 2245.548524] do_syscall_64+0x5c/0x90
[ 2245.552104] ? syscall_exit_to_user_mode+0x12/0x30
[ 2245.556897] ? do_syscall_64+0x69/0x90
[ 2245.560648] ? do_syscall_64+0x69/0x90
[ 2245.564394] entry_SYSCALL_64_after_hwframe+0x63/0xcd
[ 2245.569447] RIP: 0033:0x7fe52583ec6b
...
[ 2245.646771] ------------[ cut here ]------------
[ 2245.651395] kmem_cache_destroy bt_work: Slab cache still has objects when called from btracker_destroy+0x12/0x20 [dm_cache]
[ 2245.651408] WARNING: CPU: 7 PID: 10805 at mm/slab_common.c:478 kmem_cache_destroy+0x128/0x130

Found using: lvm2-testsuite --only "cache-single-split.sh"

Ben bisected and found that commit 0495e337b703 ("mm/slab_common:
Deleting kobject in kmem_cache_destroy() without holding
slab_mutex/cpu_hotplug_lock") first exposed dm-cache's incomplete
cleanup of its background tracker work objects.