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>
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 37d2438..abb11e3 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2253,7 +2253,7 @@
 
 	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
 	{