selftests: Consistently specify address for MD5 protection

nettest started with -r as the remote address for MD5 passwords.
The -m argument was added to use prefixes with a length when that
feature was added to the kernel. Since -r is used to specify
remote address for client mode, change nettest to only use -m
for MD5 passwords and update fcnal-test script.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>