blk-cgroup: fix UAF by grabbing blkcg lock before destroying blkg pd

KASAN reports a use-after-free report when doing fuzz test:

[693354.104835] ==================================================================
[693354.105094] BUG: KASAN: use-after-free in bfq_io_set_weight_legacy+0xd3/0x160
[693354.105336] Read of size 4 at addr ffff888be0a35664 by task sh/1453338

[693354.105607] CPU: 41 PID: 1453338 Comm: sh Kdump: loaded Not tainted 4.18.0-147
[693354.105610] Hardware name: Huawei 2288H V5/BC11SPSCB0, BIOS 0.81 07/02/2018
[693354.105612] Call Trace:
[693354.105621]  dump_stack+0xf1/0x19b
[693354.105626]  ? show_regs_print_info+0x5/0x5
[693354.105634]  ? printk+0x9c/0xc3
[693354.105638]  ? cpumask_weight+0x1f/0x1f
[693354.105648]  print_address_description+0x70/0x360
[693354.105654]  kasan_report+0x1b2/0x330
[693354.105659]  ? bfq_io_set_weight_legacy+0xd3/0x160
[693354.105665]  ? bfq_io_set_weight_legacy+0xd3/0x160
[693354.105670]  bfq_io_set_weight_legacy+0xd3/0x160
[693354.105675]  ? bfq_cpd_init+...