net: sched: cls_bpf: rename cls_bpf_modify_existing function

The name cls_bpf_modify_existing is highly misleading, as it indeed does
not modify anything existing. It does not modify at all.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>