There are no functional changes, avoid dereferencing bitmap directly to
prepare inventing a new bitmap.

Also fix following checkpatch warning by using wq_has_sleeper().

WARNING: waitqueue_active without comment
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-9-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>