bcache: use sysfs_match_string() instead of __sysfs_match_string()

The arrays (of strings) that are passed to __sysfs_match_string() are
static, so use sysfs_match_string() which does an implicit ARRAY_SIZE()
over these arrays.

Functionally, this doesn't change anything.
The change is more cosmetic.

It only shrinks the static arrays by 1 byte each.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>