IB/mlx4: Add support for MRA

Using CX-3 in virtualized mode, MAD packets are proxied through the PF
driver. However, the handling lacks support of the MRA (Message Receipt
Acknowledgment) packet. When having dynamic debug enabled, we see tons of:

mlx4_ib_multiplex_cm_handler: id{slave: 7, sl_cm_id: 0x8fcb45a0} is NULL! attr_id: 0x11

Fixes: 3cf69cc8 ("IB/mlx4: Add CM paravirtualization")
Link: https://lore.kernel.org/r/20200803061941.1139994-3-haakon.bugge@oracle.com


Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>