Impact: cleanup, small kernel text size reduction, no functionality changed

reserve_region_with_split() calls in to __reserve_region_with_split(),
which is an __init function. The only caller of reserve_region_with_split()
is an __init function, so make it __init too.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>