md/md-bitmap: make in memory structure internal

Now that struct bitmap_page and bitmap is not used externally anymore,
move them from md-bitmap.h to md-bitmap.c (expect that dm-raid is still
using define marco 'COUNTER_MAX').

Also fix some checkpatch warnings.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240826074452.1490072-43-yukuai1@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>