btrfs: use assertion instead of BUG_ON when adding/removing to delalloc list
When adding or removing and inode to/from the root's delalloc list,
instead of using a BUG_ON() to validate list emptiness, use ASSERT()
since this is to check logic errors rather than real errors.
Reviewed-by: Boris Burkov <boris@bur.io> Reviewed-by: Qu Wenruo <wqu@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>