percpu: allow non-linear / sparse cpu -> unit mapping

Currently cpu and unit are always identity mapped.  To allow more
efficient large page support on NUMA and lazy allocation for possible
but offline cpus, cpu -> unit mapping needs to be non-linear and/or
sparse.  This can be easily implemented by adding a cpu -> unit
mapping array and using it whenever looking up the matching unit for a
cpu.

The only unusal conversion is in pcpu_chunk_addr_search().  The passed
in address is unit0 based and unit0 might not be in use so it needs to
be converted to address of an in-use unit.  This is easily done by
adding the unit offset for the current processor.

[ Impact: allows non-linear/sparse cpu -> unit mapping, no visible change yet ]

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

arch/sparc/kernel/smp_64.c

@@ -1516,7 +1516,7 @@ void __init setup_per_cpu_areas(void)
 
 	pcpu_unit_size = pcpu_setup_first_chunk(static_size,
 						PERCPU_MODULE_RESERVE, dyn_size,
-						PCPU_CHUNK_SIZE, vm.addr);
+						PCPU_CHUNK_SIZE, vm.addr, NULL);
 
 	free_bootmem(__pa(ptrs), ptrs_size);