mac80211: fix RCU warnings in mesh

Sparse RCU checking reports two warnings in the mesh
path table code. These are due to questionable uses of
rcu_dereference.

To fix the first one, get rid of mesh_gate_add() and
just make mesh_path_add_gate() do the correct deref.

To fix the second one, simply remove rcu_dereference()
in mesh_gate_del() -- it already gets a proper pointer
as indicated by the prototype (no __rcu annotation)
and confirmed by the code.

Cc: Javier Cardona <javier@cozybit.com>
Cc: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>