md/md-bitmap: add a none backend for bitmap grow

md/md-bitmap: add a none backend for bitmap grow

Add a real none bitmap backend that exposes the common bitmap sysfs
group and use it to keep bitmap/location available when an array has no
bitmap.

Then switch the bitmap location sysfs path to move only between none
and the classic bitmap backend, using the no-sysfs bitmap helpers while
merging or unmerging the internal bitmap sysfs group.

This restores mdadm --grow bitmap addition through bitmap/location.

Fixes: fb8cc3b0d9db ("md/md-bitmap: delay registration of bitmap_ops until creating bitmap")
Reviewed-by: Su Yue <glass.su@suse.com>
Link: https://lore.kernel.org/r/20260425024615.1696892-4-yukuai@fnnas.com
Signed-off-by: Yu Kuai <yukuai@fnnas.com>