md: bcache: check the return value of kzalloc() in detached_dev_do_request()

The function kzalloc() in detached_dev_do_request() can fail, so its
return value should be checked.

Fixes: bc082a55

 ("bcache: fix inaccurate io state for detached bcache devices")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20220527152818.27545-4-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>