btrfs: use __u16 for the return value of btrfs_qgroup_level()

The qgroup level is limited to u16, so no need to use u64 for it.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>