ext4: fix up rb_root initializations to use RB_ROOT

ext4 uses rb_node = NULL; to zero rb_root at few places.  Using
RB_ROOT as the initializer is more portable in case the underlying
implementation of rbtrees changes in the future.
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Eric Paris <eparis@redhat.com>

fs/ext4/block_validity.c

@@ -205,14 +205,14 @@ void ext4_release_system_zone(struct super_block *sb)
 		entry = rb_entry(n, struct ext4_system_zone, node);
 		kmem_cache_free(ext4_system_zone_cachep, entry);
 		if (!parent)
-			EXT4_SB(sb)->system_blks.rb_node = NULL;
+			EXT4_SB(sb)->system_blks = RB_ROOT;
 		else if (parent->rb_left == n)
 			parent->rb_left = NULL;
 		else if (parent->rb_right == n)
 			parent->rb_right = NULL;
 		n = parent;
 	}
-	EXT4_SB(sb)->system_blks.rb_node = NULL;
+	EXT4_SB(sb)->system_blks = RB_ROOT;
 }
 
 /*

fs/ext4/dir.c

@@ -305,7 +305,7 @@ static void free_rb_tree_fname(struct rb_root *root)
 			kfree(old);
 		}
 		if (!parent)
-			root->rb_node = NULL;
+			*root = RB_ROOT;
 		else if (parent->rb_left == n)
 			parent->rb_left = NULL;
 		else if (parent->rb_right == n)

fs/ext4/mballoc.c

@@ -2253,7 +2253,7 @@ int ext4_mb_add_groupinfo(struct super_block *sb, ext4_group_t group,
 
 	INIT_LIST_HEAD(&meta_group_info[i]->bb_prealloc_list);
 	init_rwsem(&meta_group_info[i]->alloc_sem);
-	meta_group_info[i]->bb_free_root.rb_node = NULL;
+	meta_group_info[i]->bb_free_root = RB_ROOT;
 
 #ifdef DOUBLE_CHECK
 	{