if_ntb: MFV 2849b5d7: Reset transport QP link stats on down

Reset the link stats when the link goes down. In particular, the TX and
RX index and count must be reset, or else the TX side will be sending
packets to the RX side where the RX side is not expecting them. Reset
all the stats, to be consistent.

Authored by: Allen Hubbe
Obtained from: Linux (Dual BSD/GPL driver)
Sponsored by: EMC / Isilon Storage Division