RDMA/usnic: Suppress a compiler warning

This patch avoids that the following compiler warning is reported when
building with gcc 8 and W=1:

drivers/infiniband/hw/usnic/usnic_fwd.c:95:2: warning: 'strncpy' output may be truncated copying 16 bytes from a string of length 20 [-Wstringop-truncation]
  strncpy(ufdev->name, netdev_name(ufdev->netdev),
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    sizeof(ufdev->name) - 1);
    ~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>