net: cls_bpf: fix NULL deref on offload filter removal
    
    Commit 401192113730 ("net: sched: refactor block offloads counter
    usage") missed the fact that either new prog or old prog may be
    NULL.
    
    Fixes: 401192113730 ("net: sched: refactor block offloads counter usage")
    Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>