btrfs: root-tree: remove unnecessary calls to btrfs_mark_buffer_dirty()

We have several places explicitly calling btrfs_mark_buffer_dirty() but
that is not necessarily since the target leaf came from a path that was
obtained for a btree search function that modifies the btree, something
like btrfs_insert_empty_item() or anything else that ends up calling
btrfs_search_slot() with a value of 1 for its 'cow' argument.

These just make the code more verbose, confusing and add a little extra
overhead and well as increase the module's text size, so remove them.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/root-tree.c

@@ -197,7 +197,6 @@ int btrfs_update_root(struct btrfs_trans_handle *trans, struct btrfs_root
 	btrfs_set_root_generation_v2(item, btrfs_root_generation(item));
 
 	write_extent_buffer(l, item, ptr, sizeof(*item));
-	btrfs_mark_buffer_dirty(trans, path->nodes[0]);
 out:
 	btrfs_free_path(path);
 	return ret;
@@ -447,7 +446,6 @@ int btrfs_add_root_ref(struct btrfs_trans_handle *trans, u64 root_id,
 	btrfs_set_root_ref_name_len(leaf, ref, name->len);
 	ptr = (unsigned long)(ref + 1);
 	write_extent_buffer(leaf, name->name, ptr, name->len);
-	btrfs_mark_buffer_dirty(trans, leaf);
 
 	if (key.type == BTRFS_ROOT_BACKREF_KEY) {
 		btrfs_release_path(path);