include/linux/memblock.h: add __init to memblock_set_bottom_up()

memblock_set_bottom_up() is only called by __init
cmdline_parse_movable_node() and __init numa_init().

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Reviewed-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>