Btrfs: remove superfluous NULL pointer check in btrfs_rename()

This takes care of the following entry from Dan's list:

fs/btrfs/inode.c +4788 btrfs_rename(36) warning: variable derefenced before check 'old_inode'
Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Eugene Teo <eteo@redhat.com>
Cc: Julia Lawall <julia@diku.dk>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/inode.c

@@ -4806,8 +4806,7 @@ static int btrfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 	 * and the replacement file is large.  Start IO on it now so
 	 * we don't add too much work to the end of the transaction
 	 */
-	if (new_inode && old_inode && S_ISREG(old_inode->i_mode) &&
-	    new_inode->i_size &&
+	if (new_inode && S_ISREG(old_inode->i_mode) && new_inode->i_size &&
 	    old_inode->i_size > BTRFS_ORDERED_OPERATIONS_FLUSH_LIMIT)
 		filemap_flush(old_inode->i_mapping);