EX-3937 lnet: check if ethtool.h is included in netdevice.h

EX-3937 lnet: check if ethtool.h is included in netdevice.h

Kernel 5.11.0-36 removes including linux/ethtool.h from
linux/netdevice.h, which caused the following build error:

dereferencing pointer to incomplete type ‘const struct ethtool_ops’

This patch fixes the above issue by checking if ethtool.h
is included in netdevice.h.

Change-Id: Ifc25de5acaebf2b5fd5bb6f1c303366ab9ea6ef6
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/45097
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>