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>

arch/sparc/kernel/smp_64.c

@@ -1528,7 +1528,7 @@ void __init setup_per_cpu_areas(void)
 
 	pcpu_unit_size = pcpu_setup_first_chunk(pcpur_get_page, static_size,
 						PERCPU_MODULE_RESERVE, dyn_size,
-						PCPU_CHUNK_SIZE, vm.addr, NULL);
+						PCPU_CHUNK_SIZE, vm.addr);
 
 	free_bootmem(__pa(pcpur_ptrs), ptrs_size);