RDMA/core: Add support to set privileged QKEY parameter

Add netlink command that enables/disables privileged QKEY by default.

It is disabled by default, since according to IB spec only privileged
users are allowed to use privileged QKEY.
According to the IB specification rel-1.6, section 3.5.3:
"QKEYs with the most significant bit set are considered controlled
QKEYs, and a HCA does not allow a consumer to arbitrarily specify a
controlled QKEY."

Using rdma tool,
$rdma system set privileged-qkey on

When enabled non-privileged users would be able to use
controlled QKEYs which are considered privileged.

Using rdma tool,
$rdma system set privileged-qkey off

When disabled only privileged users would be able to use
controlled QKEYs.

You can also use the command below to check the parameter state:
$rdma system show
netns shared privileged-qkey off copy-on-fork on

Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Link: https://lore.kernel.org/r/90398be70a9d23d2aa9d0f9fd11d2c264c1be534.1696848201.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>