Fix incorrect MapgenV6 chunk size warning

src/mapgen/mapgen.h

@@ -165,6 +165,7 @@ public:
 	u32 blockseed;
 	s16 *heightmap = nullptr;
 	biome_t *biomemap = nullptr;
+	// Chunk size in nodes
 	v3s16 csize;
 
 	BiomeGen *biomegen = nullptr;

src/mapgen/mapgen_v6.cpp

@@ -49,7 +49,7 @@ MapgenV6::MapgenV6(MapgenV6Params *params, EmergeParams *emerge)
 		// (5,2,5) generates very broken terrain
 		throw BaseException("MapgenV6: chunk size must be cubic");
 	}
-	if (csize.Y % 2 == 0) {
+	if (csize.Y % (MAP_BLOCKSIZE * 2) == 0) {
 		// weird ledges appear in some places
 		warningstream << "MapgenV6: chunk heights divisible by two are known "
 			"to be buggy." << std::endl;