nettest: Add support for TCP_MD5 extensions

Update nettest to implement TCP_MD5SIG_EXT for a prefix and a device.

Add a new option, -m, to specify a prefix and length to use with MD5
auth. The device option comes from the existing -d option. If either
are set and MD5 auth is requested, TCP_MD5SIG_EXT is used instead of
TCP_MD5SIG.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>