percpu: simplify pcpu_setup_first_chunk()

Now that all first chunk allocator helpers allocate and map the first
chunk themselves, there's no need to have optional default alloc/map
in pcpu_setup_first_chunk().  Drop @populate_pte_fn and only leave
@dyn_size optional and make all other params mandatory.

This makes it much easier to follow what pcpu_setup_first_chunk() is
doing and what actual differences tweaking each parameter results in.

[ Impact: drop unused code path ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>