mac80211: return new mpath from mesh_path_add()

Most times that mesh_path_add() is called, it is followed by
a lookup to get the just-added mpath.  We can instead just
return the new mpath in the case that we allocated one (or the
existing one if already there), so do that.  Also, reorder the
code in mesh_path_add a bit so that we don't need to allocate
in the pre-existing case.

Signed-off-by: Bob Copeland <bob@cozybit.com>