memory hotplug x86_64: fix section mismatch in init_memory_mapping()

Changes __meminit to __init_refok.

WARNING: vmlinux.o(.text+0x1d07c): Section mismatch: reference to
.init.text:find_e820_area (between 'init_memory_mapping' and 'arch_add_memory')

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>