The area map allocator only used a bitmap for the backing page state.
The new bitmap allocator will use bitmaps to manage the allocation
region in addition to this.

This patch generalizes the bitmap iterators so they can be reused with
the bitmap allocator.
Signed-off-by: NDennis Zhou <dennisszhou@gmail.com>
Reviewed-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NTejun Heo <tj@kernel.org>