btrfs: fix leak of path in btrfs_find_item

commit 381cf6587f8a8a8e981bc0c1aaaa8859b51dc756 upstream.

If btrfs_find_item is called with NULL path it allocates one locally but
does not free it. Affected paths are inserting an orphan item for a file
and for a subvol root.

Move the path allocation to the callers.

Fixes: 3f870c28 ("btrfs: expand btrfs_find_item() to include find_orphan_item functionality")
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>