m_bpf: don't require default opcode on ebpf actions

m_bpf: don't require default opcode on ebpf actions

After the patch, the most minimal command to load an eBPF action
for late binding with auto index selection through tc is:

  tc actions add action bpf obj prog.o

We already set TC_ACT_PIPE in tc as default opcode, so if nothing
further has been specified, just use it. Also, allow "ok" next to
"pass" for matching cmdline on TC_ACT_OK.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>