selftests: net: fix cmsg_so_mark.sh test hang

This cmsg_so_mark.sh test will hang on non-amd64 systems because of the
infinity loop for argument parsing in cmsg_sender.

Variable "o" in cs_parse_args() for taking getopt() should be an int,
otherwise it will be 255 when getopt() returns -1 on non-amd64 system
and thus causing infinity loop.

Link: https://lore.kernel.org/lkml/CA+G9fYsM2k7mrF7W4V_TrZ-qDauWM394=8yEJ=-t1oUg8_40YA@mail.gmail.com/t/
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>