mm/memblock.c: fix index adjustment error in __next_mem_range_rev()

Fix region index adjustment error when parameter type_b of
__next_mem_range_rev() == NULL.
Signed-off-by: zijun_hu <zijun_hu@htc.com>
Cc: Alexander Kuleshov <kuleshovmail@gmail.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Wei Yang <weiyang@linux.vnet.ibm.com>
Cc: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Richard Leitner <dev@g0hl1n.net>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memblock.c

@@ -1027,7 +1027,7 @@ void __init_memblock __next_mem_range_rev(u64 *idx, int nid, ulong flags,
 				*out_end = m_end;
 			if (out_nid)
 				*out_nid = m_nid;
-			idx_a++;
+			idx_a--;
 			*idx = (u32)idx_a | (u64)idx_b << 32;
 			return;
 		}