md-bitmap: create and destroy wb_info_pool with the change of bitmap

The write-behind attribute is part of bitmap, since bitmap
can be added/removed dynamically with the following.

1. mdadm --grow /dev/md0 --bitmap=none
2. mdadm --grow /dev/md0 --bitmap=internal --write-behind

So we need to destroy wb_info_pool in md_bitmap_destroy,
and create the pool before load bitmap.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Song Liu <songliubraving@fb.com>