ethtool: Report link-down while interface is down

ethtool: Report link-down while interface is down

While an interface is down, many implementations of
ethtool_ops::get_link, including the default, ethtool_op_get_link(),
will report the last link state seen while the interface was up.  In
general the current physical link state is not available if the
interface is down.

Define ETHTOOL_GLINK to reflect whether the interface *and* any
physical port have a working link, and consistently return 0 when the
interface is down.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit e596e6e4d578f2639416e620d367a3af34814a40)
Cc: Wang Weidong <wangweidong1@huawei.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>