nettest: Return 1 on MD5 failure for server mode

On failure to set MD5 password, do_server should return 1 so that the
program exits with 1 rather than 255. This used for negative testing
when adding MD5 with device option.

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