samples/bpf: Fix incorrect use of strlen in xdp_redirect_cpu

Commit b599015f ("samples/bpf: Fix application of sizeof to pointer")
tried to fix a bug where sizeof was incorrectly applied to a pointer instead
of the array string was being copied to, to find the destination buffer size,
but ended up using strlen, which is still incorrect. However, on closer look
ifname_buf has no other use, hence directly use optarg.

Fixes: b599015f ("samples/bpf: Fix application of sizeof to pointer")
Fixes: e531a220

 ("samples: bpf: Convert xdp_redirect_cpu to XDP samples helper")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Tested-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Link: https://lore.kernel.org/bpf/20211112020301.528357-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>