net: don't include ethtool.h from netdevice.h

linux/netdevice.h is included in very many places, touching any
of its dependecies causes large incremental builds.

Drop the linux/ethtool.h include, linux/netdevice.h just needs
a forward declaration of struct ethtool_ops.

Fix all the places which made use of this implicit include.

Acked-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Shannon Nelson <snelson@pensando.io>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/20201120225052.1427503-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
diff --git a/net/socket.c b/net/socket.c
index 152b1dc..bfef11b 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -52,6 +52,7 @@
  *	Based upon Swansea University Computer Society NET3.039
  */
 
+#include <linux/ethtool.h>
 #include <linux/mm.h>
 #include <linux/socket.h>
 #include <linux/file.h>