mm/bootmem.c: properly __init-annotate helper functions

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>