percpu: setup_first_chunk enforce dynamic region must exist

The first chunk is handled as a special case as it is composed of the
static, reserved, and dynamic regions. The code handles each case
individually. The next several patches will merge these code paths and
lay the foundation for the bitmap allocator.

This patch modifies logic to enforce that a dynamic region exists and
changes the area map to account for that. This brings the logic closer
to the dynamic chunk's init logic.

Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>