IB/core: Add query_port_speed verb

IB/core: Add query_port_speed verb

Add new ibv_query_port_speed() verb to enable applications to query
the effective bandwidth of a port.

This verb is particularly useful when the speed is not a multiplication
of IB speed and width where width is 2^n.

Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>