LU-15052 lnet: include linux/ethtool.h

LU-15052 lnet: include linux/ethtool.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 adding the include into
the file that uses the structure.

Test-Parameters: trivial

Change-Id: Ifc25de5acaebf2b5fd5bb6f1c303366ab9ea6ef6
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/45109
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>