gve: Fixes for napi_poll when budget is 0

gve: Fixes for napi_poll when budget is 0

Netpoll will explicilty pass the polling call with a budget of 0 to
indicate it's clearing the Tx path only. For the gve_rx_poll and
gve_xdp_poll, they were mistakenly taking the 0 budget as the indication
to do all the work. Add check to avoid the rx path and xdp path being
called when budget is 0. And also avoid napi_complete_done being called
when budget is 0 for netpoll.

Fixes: f5cedc84a30d ("gve: Add transmit and receive support")
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Link: https://lore.kernel.org/r/20231114004144.2022268-1-ziweixiao@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>