net/ena: refactor ena_get_stats64 to be atomic context safe

net/ena: refactor ena_get_stats64 to be atomic context safe

BugLink: http://bugs.launchpad.net/bugs/1664312
ndo_get_stat64() can be called from atomic context, but the current
implementation sends an admin command to retrieve the statistics from
the device. This admin command can sleep.

This patch re-factors the implementation of ena_get_stats64() to use
the {rx,tx}bytes/count from the driver's inner counters, and to obtain
the rx drop counter from the asynchronous keep alive (heart bit)
event.

Signed-off-by: Netanel Belgazal <netanel@annapurnalabs.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[kamal: backport from commit d81db24056132fe8b83e2fba337e9ea76675e68d (context)]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>