Add a proc interface for max_dir_size.

Index: linux-3.10.0-123.13.2.el7.x86_64/fs/ext4/sysfs.c
===================================================================
--- linux-3.10.0-123.13.2.el7.x86_64.orig/fs/ext4/sysfs.c
+++ linux-3.10.0-123.13.2.el7.x86_64/fs/ext4/sysfs.c
@@ -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),
@@ -2676,7 +2476,9 @@ static ssize_t sbi_ui_show(struct ext4_a
		if (!ptr)
			return 0;
		return snprintf(buf, PAGE_SIZE, "%u\n",
-				*((unsigned int *) ptr));
+				strcmp("max_dir_size", a->attr.name) ?
+				*((unsigned int *) ptr) :
+				(*((unsigned int *) ptr)) << 10);
	case attr_pointer_atomic:
		if (!ptr)
			return 0;
@@ -2691,6 +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;
		*((unsigned int *) ptr) = t;
		return len;
	case attr_inode_readahead: