f_bpf: fix filling of handle when no further arg is provided

f_bpf: fix filling of handle when no further arg is provided

We need to fill handle when provided by the user, even if no further
argument is provided. Thus, move the test for arg to the correct location,
so that it works correctly:

  # tc filter show dev foo egress
  filter protocol all pref 1 bpf
  filter protocol all pref 1 bpf handle 0x1 bpf.o:[classifier] direct-action
  filter protocol all pref 1 bpf handle 0x2 bpf.o:[classifier] direct-action
  # tc filter del dev foo egress prio 1 handle 2 bpf
  # tc filter show dev foo egress
  filter protocol all pref 1 bpf
  filter protocol all pref 1 bpf handle 0x1 bpf.o:[classifier] direct-action

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