sendmsg01: use invalid but positive msg_namelen value

After following 2 kernel commits:
  commit 1661bf364ae9c506bc8795fef70d1532931be1e8
  Author: Dan Carpenter <dan.carpenter@oracle.com>
  Date:   Thu Oct 3 00:27:20 2013 +0300
    net: heap overflow in __audit_sockaddr()

  commit db31c55a6fb245fdbb752a2ca4aefec89afabb06
  Author: Dan Carpenter <dan.carpenter@oracle.com>
  Date:   Wed Nov 27 15:40:21 2013 +0300
    net: clamp ->msg_namelen instead of returning an error

msg_namelen is treated as an unsigned value because of this
condition, which compares signed and unsigned arguments:
net/socket.c copy_msghdr_from_user()
  if (kmsg->msg_namelen > sizeof(struct sockaddr_storage))

User-space (accordding to POSIX spec) defines it as
"unsigned opaque integral type of length of at least 32 bits".

Passing -1 now has the effect as passing very large number
and syscall completes successfully.

Change the test to use invalid, but positive value for
"invalid to buffer length" testcase.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>