datapath: Eliminate synchronize_rcu() in port group update.

datapath: Eliminate synchronize_rcu() in port group update.

We found out some time ago that synchronize_rcu() can block for multiple
seconds in some cases, so it's a good idea to eliminate as many of them
as we can.

This removes such a call in set_port_group().  This requires adding an
rcu_head to the data structure for port groups; since until now we've just
used the same "struct odp_port_group" exported to userspace, this means
that we need to introduce a new "struct dp_port_group" for internal use,
which in turn causes a fair bit of code motion.

Bug #1233.