net: ena: fix napi handler misbehavior when the napi budget is zero

[ Upstream commit 24dee0c7 ]

In netpoll the napi handler could be called with budget equal to zero.
Current ENA napi handler doesn't take that into consideration.

The napi handler handles Rx packets in a do-while loop.
Currently, the budget check happens only after decrementing the
budget, therefore the napi handler, in rare cases, could run over
MAX_INT packets.

In addition to that, this moves all budget related variables to int
calculation and stop mixing u32 to avoid ambiguity

Fixes: 1738cd3e ("net: ena: Add a driver for Amazon Elastic Network Adapters (ENA)")
Signed-off-by: Netanel Belgazal <netanel@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>