memblock: Fix section mismatch warnings

Stephen found a bunch of section mismatch warnings with the
new memblock changes.

Use __init_memblock to replace __init in memblock.c and remove
__init in memblock.h. We should not use __init in header files.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Yinghai Lu <Yinghai@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
LKML-Reference: <4C912709.2090201@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>