Add return value to reserve_bootmem_node()

This patch changes the function reserve_bootmem_node() from void to int,
returning -ENOMEM if the allocation fails.

This fixes a build problem on x86 with CONFIG_KEXEC=y and
CONFIG_NEED_MULTIPLE_NODES=y

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>