Add a proc interface for max_dir_size.

Index: linux-3.10.0-123.13.2.el7.x86_64/fs/ext4/super.c
===================================================================
--- linux-3.10.0-123.13.2.el7.x86_64.orig/fs/ext4/super.c
+++ linux-3.10.0-123.13.2.el7.x86_64/fs/ext4/super.c
@@ -2476,8 +2476,11 @@ static ssize_t sbi_ui_show(struct ext4_a
 			   struct ext4_sb_info *sbi, char *buf)
 {
 	unsigned int *ui = (unsigned int *) (((char *) sbi) + a->u.offset);
+	unsigned int v = *ui;
 
-	return snprintf(buf, PAGE_SIZE, "%u\n", *ui);
+	if (strcmp("max_dir_size", a->attr.name) == 0)
+		v <<= 10;
+	return snprintf(buf, PAGE_SIZE, "%u\n", v);
 }
 
 static ssize_t sbi_ui_store(struct ext4_attr *a,
@@ -2491,6 +2494,8 @@ static ssize_t sbi_ui_store(struct ext4_
 	ret = kstrtoul(skip_spaces(buf), 0, &t);
 	if (ret)
 		return ret;
+	if (strcmp("max_dir_size", a->attr.name) == 0)
+		t >>= 10;
 	*ui = t;
 	return count;
 }
@@ -2557,6 +2562,8 @@ EXT4_RW_ATTR(reserved_clusters);
 EXT4_ATTR_OFFSET(inode_readahead_blks, 0644, sbi_ui_show,
 		 inode_readahead_blks_store, s_inode_readahead_blks);
 EXT4_RW_ATTR_SBI_UI(inode_goal, s_inode_goal);
+EXT4_RW_ATTR_SBI_UI(max_dir_size, s_max_dir_size_kb);
+EXT4_RW_ATTR_SBI_UI(max_dir_size_kb, s_max_dir_size_kb);
 EXT4_RW_ATTR_SBI_UI(mb_stats, s_mb_stats);
 EXT4_RW_ATTR_SBI_UI(mb_max_to_scan, s_mb_max_to_scan);
 EXT4_RW_ATTR_SBI_UI(mb_min_to_scan, s_mb_min_to_scan);
@@ -2581,6 +2588,8 @@ static struct attribute *ext4_attrs[] =
 	ATTR_LIST(reserved_clusters),
 	ATTR_LIST(inode_readahead_blks),
 	ATTR_LIST(inode_goal),
+	ATTR_LIST(max_dir_size),
+	ATTR_LIST(max_dir_size_kb),
 	ATTR_LIST(mb_stats),
 	ATTR_LIST(mb_max_to_scan),
 	ATTR_LIST(mb_min_to_scan),