x86, memblock: Move mem_hole_size() to .init

mem_hole_size() is being called only from __init-marked functions, and as
such should be moved to .init section as well. Fixes this warning:

WARNING: vmlinux.o(.text+0x35511): Section mismatch in reference from the function mem_hole_size() to the function .init.text:absent_pages_in_range()

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Link: http://lkml.kernel.org/r/alpine.LNX.2.00.1202281614450.31150@pobox.suse.cz
Signed-off-by: H. Peter Anvin <hpa@zytor.com>