For all string attributes for which we don't currently accept the element
as input, we only use it as output, set the string length to
RDMA_NLDEV_ATTR_EMPTY_STRING which is defined as 1.  That way we will only
accept a null string for that element.  This will prevent someone from
writing a new input routine that uses the element without also updating
the policy to have a valid value.

Also while there, make sure the existing entries that are valid have the
correct policy, if not, correct the policy.  Remove unnecessary checks
for nla_strlcpy() overflow once the policy has been set correctly.

Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>