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/dir.c b/fs/ext4/dir.c
index 29857dd..86cb6d8 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -305,7 +305,7 @@
 			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)