samples: bpf: Fix uninitialized variable in xdp_redirect_cpu

While at it, also improve help output when CPU number is greater than
possible.

Fixes: e531a220 ("samples: bpf: Convert xdp_redirect_cpu to XDP samples helper")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210826120910.454081-1-memxor@gmail.com