net: ethtool: clear heap allocations for ethtool function

[ Upstream commit 80ec82e3 ]

Several ethtool functions leave heap uncleared (potentially) by
drivers. This will leave the unused portion of heap unchanged and
might copy the full contents back to userspace.

Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>