LinuxKPI: add NETIF_F_HW_CSUM to netdev_features.h

Add NETIF_F_HW_CSUM to netdev_features.h as needed by a driver.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D34233

(cherry picked from commit 85d61bd872a0d91173d6cc8dc5bb2c927a5a4302)