f2fs: allocate node and hot data in the beginning of partition

In order to give more spatial locality, this patch changes the block allocation
policy which assigns beginning of partition for small and hot data/node blocks.
In order to do this, we set noheap allocation by default and introduce another
mount option, heap, to reset it back.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -172,7 +172,11 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
 	if (gc_type != FG_GC && p->max_search > sbi->max_victim_search)
 		p->max_search = sbi->max_victim_search;
 
-	p->offset = sbi->last_victim[p->gc_mode];
+	/* let's select beginning hot/small space first */
+	if (type == CURSEG_HOT_DATA || IS_NODESEG(type))
+		p->offset = 0;
+	else
+		p->offset = sbi->last_victim[p->gc_mode];
 }
 
 static unsigned int get_max_cost(struct f2fs_sb_info *sbi,