mana: refill the rx mbuf in batch

Set the default refill threshod to be one quarter of the rx queue
length. User can change this value with hw.mana.rx_refill_thresh
in loader.conf. It improves the rx completion handling by saving
10% to 15% of overall time with this change.

Tested by: whu
MFC after: 2 weeks
Sponsored by: Microsoft