ext4: fix up a undefined error in ext4_free_blocks in debugging code

sbi is not defined, so let ext4_free_blocks use EXT4_SB(sb) instead
when EXT4FS_DEBUG is defined.
Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>

fs/ext4/balloc.c

@@ -565,7 +565,7 @@ ext4_fsblk_t ext4_count_free_clusters(struct super_block *sb)
 	brelse(bitmap_bh);
 	printk(KERN_DEBUG "ext4_count_free_clusters: stored = %llu"
 	       ", computed = %llu, %llu\n",
-	       EXT4_B2C(sbi, ext4_free_blocks_count(es)),
+	       EXT4_B2C(EXT4_SB(sb), ext4_free_blocks_count(es)),
 	       desc_count, bitmap_count);
 	return bitmap_count;
 #else