Btrfs: don't preallocate metadata blocks during btrfs_search_slot

In order to avoid doing expensive extent management with tree locks held,
btrfs_search_slot will preallocate tree blocks for use by COW without
any tree locks held.

A later commit moves all of the extent allocation work for COW into
a delayed update mechanism, and this preallocation will no longer be
required.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/transaction.c

@@ -463,7 +463,7 @@ int btrfs_commit_tree_roots(struct btrfs_trans_handle *trans,
 	btrfs_extent_post_op(trans, fs_info->tree_root);
 
 	eb = btrfs_lock_root_node(fs_info->tree_root);
-	btrfs_cow_block(trans, fs_info->tree_root, eb, NULL, 0, &eb, 0);
+	btrfs_cow_block(trans, fs_info->tree_root, eb, NULL, 0, &eb);
 	btrfs_tree_unlock(eb);
 	free_extent_buffer(eb);
 
@@ -766,7 +766,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	btrfs_set_key_type(&key, BTRFS_ROOT_ITEM_KEY);
 
 	old = btrfs_lock_root_node(root);
-	btrfs_cow_block(trans, root, old, NULL, 0, &old, 0);
+	btrfs_cow_block(trans, root, old, NULL, 0, &old);
 
 	btrfs_copy_root(trans, root, old, &tmp, objectid);
 	btrfs_tree_unlock(old);