RDMA/nldev: Don't expose unsafe global rkey to regular user

Unsafe global rkey is considered dangerous because it exposes memory
registered for all memory in the system. Only users with a QP on the same
PD can use the rkey, and generally those QPs will already know the
value. However, out of caution, do not expose the value to unprivleged
users on the local system. Require CAP_NET_ADMIN instead.

Cc: <stable@vger.kernel.org> # 4.16
Fixes: 29cf1351 ("RDMA/nldev: provide detailed PD information")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>