f_bpf: allow for optional classid and add flags

f_bpf: allow for optional classid and add flags

When having optional classid, most minimal command can be sth
like:

  tc filter add dev foo parent X: bpf obj prog.o

Therefore, adapt the code so that a next argument will not be
enforced as the case currently.

Also, minor cleanup on the classid, where we should rather
have used addattr32(), and add flags for exec configuration,
for example (using short notation):

  tc filter add dev foo parent X: bpf da obj prog.o

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>