btrfs: use BTRFS_DIR_START_INDEX at btrfs_create_new_inode()

We are still using the magic value of 2 at btrfs_create_new_inode(), but
there's now a constant for that, named BTRFS_DIR_START_INDEX, which was
introduced in commit 528ee697 ("btrfs: put initial index value of a
directory in a constant"). So change that to use the constant.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -6227,12 +6227,8 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 		if (ret)
 			goto out;
 	}
-	/*
-	 * index_cnt is ignored for everything but a dir,
-	 * btrfs_set_inode_index_count has an explanation for the magic
-	 * number
-	 */
-	BTRFS_I(inode)->index_cnt = 2;
+	/* index_cnt is ignored for everything but a dir. */
+	BTRFS_I(inode)->index_cnt = BTRFS_DIR_START_INDEX;
 	BTRFS_I(inode)->generation = trans->transid;
 	inode->i_generation = BTRFS_I(inode)->generation;