block: fix use-after-free in __cpuhp_state_remove_instance

We got issue when remove device mapper as follows:
dmsetup create test --table "0 1048576 linear /dev/sda 0"
dmsetup suspend --nolockfs test
dmsetup load test --table "0 1048576 error /dev/sda 0"
dmsetup resume test
dmsetup suspend --nolockfs test
dmsetup load test --table "0 1048576 linear /dev/sda 0"
dmsetup resume test
dmsetup remove test
==================================================================
BUG: KASAN: wild-memory-access in __cpuhp_state_remove_instance+0x1ea/0x310
Write of size 8 at addr dead000000000122 by task dmsetup/1969

CPU: 1 PID: 1969 Comm: dmsetup Not tainted 5.18.0-next-20220524 torvalds#225
Call Trace:
 <TASK>
 dump_stack_lvl+0x6e/0x91
 print_report.cold+0x494/0x6b7
 kasan_report+0xa9/0x120
 __cpuhp_state_remove_instance+0x1ea/0x310
 bioset_exit+0x45/0x2e0
 cleanup_mapped_device+0x4f/0x190
 __dm_destroy+0x230/0x3c0
 dev_remove+0x18b/0x1d0
 ctl_ioctl+0x3d8/0x7c0
 dm_ctl_ioctl+0x21/0x30
 __x64_sys_ioctl+0x12c/0x170
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x7f0d9aa7a8d7
RSP: 002b:00007fff1ecedce8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f0d9ad5ff50 RCX: 00007f0d9aa7a8d7
RDX: 000055c98c17d130 RSI: 00000000c138fd04 RDI: 0000000000000003
RBP: 00007f0d9ad9a503 R08: 00007f0d9ad9b040 R09: 00007fff1ecedc10
R10: 0000000000000006 R11: 0000000000000246 R12: 000055c98c17d130
R13: 00007f0d9ad9a503 R14: 000055c98c17d050 R15: 00007f0d9ad9a503
==================================================================

As load different table may lead to bioset flag changes. But bio_alloc_cache_destroy
to free cache depends on if cache is NULL, and also this after free cache didn't
set 'bs->cache' NULL. So this will lead to UAF.
To solve above issue just set ''bs->cache' NULL after free cache.

Fixes: be4d234("bio: add allocation cache abstraction")
Signed-off-by: Ye Bin <yebin10@huawei.com>

block/bio.c

@@ -722,6 +722,7 @@ static void bio_alloc_cache_destroy(struct bio_set *bs)
 		bio_alloc_cache_prune(cache, -1U);
 	}
 	free_percpu(bs->cache);
+	bs->cache = NULL;
 }
 
 /**