mac80211: mesh: always use the latest target_sn

When a path target responds to a path request, its response
always contains the most up-to-date information; accordingly,
it should use the latest target_sn, regardless of
net_traversal_jiffies().  Otherwise, only the first path
response is considered when constructing a path, as it will
have the highest target_sn of all replies during that period.

Signed-off-by: Bob Copeland <bob@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>