ANDROID: GKI: device.h: add rcu_head back to struct device_link

Commit 80dd33cf ("drivers: base: Fix device link removal") upstream
drops the rcu_head from struct device_link.  But we want to drag it back
here in the Android GKI kernel to be 'safe' for future changes where we
might want to switch back to the old method if testing does not work out
well.

This allows us to switch between the different methods in an ABI
compatible way.

Bug: 161946584
Reported-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: If4a56bae6eebe3de5764052b603162071e81bce3