bnxt_en: Fix race conditions in .ndo_get_stats64().

.ndo_get_stats64() may not be protected by RTNL and can race with
.ndo_stop() or other ethtool operations that can free the statistics
memory.  Fix it by setting a new flag BNXT_STATE_READ_STATS and then
proceeding to read statistics memory only if the state is OPEN.  The
close path that frees the memory clears the OPEN state and then waits
for the BNXT_STATE_READ_STATS to clear before proceeding to free the
statistics memory.

Fixes: c0c050c5 ("bnxt_en: New Broadcom ethernet driver.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>