gve: fix gve_get_stats()

gve_get_stats() can report wrong numbers if/when u64_stats_fetch_retry()
returns true.

What is needed here is to sample values in temporary variables,
and only use them after each loop is ended.

Fixes: f5cedc84

 ("gve: Add transmit and receive support")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Catherine Sullivan <csully@google.com>
Cc: Sagi Shahar <sagis@google.com>
Cc: Jon Olson <jonolson@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Luigi Rizzo <lrizzo@google.com>
Cc: Jeroen de Borst <jeroendb@google.com>
Cc: Tao Liu <xliutaox@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>