mac80211: mesh: embed gates hlist head directly

Since we have converted the mesh path tables to rhashtable, we are
no longer swapping out the entire mesh_pathtbl pointer with RCU.
As a result, we no longer need indirection to the hlist head for
the gates list and can simply embed it, saving a pair of
pointer-sized allocations.

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