ethtool: fix missing NLM_F_MULTI flag when dumping

When dumping the ethtool information from all the interfaces, the
netlink reply should contain the NLM_F_MULTI flag. This flag allows
userspace tools to identify that multiple messages are expected.

Link: https://bugzilla.redhat.com/1953847
Fixes: 365f9ae4

 ("ethtool: fix genlmsg_put() failure handling in ethnl_default_dumpit()")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: David S. Miller <davem@davemloft.net>