md: factor bitmap creation away from sysfs handling

Factor bitmap creation and destruction into helpers that do not touch
bitmap sysfs registration.

This prepares the bitmap sysfs rework so callers such as the sysfs
bitmap location path can create or destroy a bitmap backend without
coupling that to sysfs group lifetime management.

Reviewed-by: Su Yue <glass.su@suse.com>
Link: https://lore.kernel.org/r/20260425024615.1696892-2-yukuai@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fnnas.com>

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