mac80211: Fix incorrectly named last_hop_metric variable in mesh_rx_path_sel_frame

The last hop metric should refer to link cost (this is how
hwmp_route_info_get uses it for example). But in mesh_rx_path_sel_frame
we are not dealing with link cost but with the total cost to the origin
of a PREQ or PREP.

Signed-off-by: Alexis Green <agreen@cococorp.com>
CC: Jesse Jones <jjones@cococorp.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>