mana: Add RX fencing

RX fencing allows the driver to know that any prior change to the RQs has
finished, e.g. when the RQs are disabled/enabled or the hashkey/indirection
table are changed, RX fencing is required.

Remove the previous 'sleep' workaround and add the real support for
RX fencing as the PF driver supports the MANA_FENCE_RQ request now (any
old PF driver not supporting the request won't be used in production).

MFC after: 2 weeks
Sponsored by: Microsoft