rdma: Add the prefix for driver attributes

There is a need to distinguish between driver vs. general exposed
attributes. The most common use case is to expose some internal
garbage under extremely common and sexy name, e.g. pi, ci e.t.c

In order to achieve that, we will add "drv_" prefix to all strings
which were received through RDMA_NLDEV_ATTR_DRIVER_* attributes.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>a
Tested-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: David Ahern <dsahern@gmail.com>