IB/mlx5: Add DEVX query EQN support

Return the matching device EQN for a given user vector number via the
DEVX interface.

Note:
EQs are owned by the kernel and shared by all user processes.
Basically, a user CQ can point to any EQ.
The kernel doesn't enforce any such limitation today either.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>