kmemleak: Add callbacks to the bootmem allocator

This patch adds kmemleak_alloc/free callbacks to the bootmem allocator.
This would allow scanning of such blocks and help avoiding a whole class
of false positives and more kmemleak annotations.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>