And move the condition "if (mddev->bitmap)" into md_bitmap_resize() as
well, on the one hand make code cleaner, on the other hand try not to
access bitmap directly.

Since we are here, also change the parameter 'init' from int to bool.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-35-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>