ibmvnic: Do not replenish RX buffers after every polling loop

ibmvnic: Do not replenish RX buffers after every polling loop

Reduce the amount of time spent replenishing RX buffers by only doing
so once available buffers has fallen under a certain threshold, in this
case half of the total number of buffers, or if the polling loop exits
before the packets processed is less than its budget. Non-exhaustion of
NAPI budget implies lower incoming packet pressure, allowing the leeway
to refill the buffers in preparation for any impending burst.

Signed-off-by: Dwip N. Banerjee <dnbanerg@us.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>