bpf, samples: Add missing mprog-disable to xdp_redirect_cpu's optstring

Commit ce4dade7 ("samples/bpf: xdp_redirect_cpu: Load a eBPF program
on cpumap") added the following option, but missed adding it to optstring:

  - mprog-disable: disable loading XDP program on cpumap entries

Fix it and add the missing option character.

Fixes: ce4dade7 ("samples/bpf: xdp_redirect_cpu: Load a eBPF program on cpumap")
Signed-off-by: Matthew Cover <matthew.cover@stackpath.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210731005632.13228-1-matthew.cover@stackpath.com