ovs-vswitchd makes extensive use of RCU to defer freeing memory past the
latest time that it could be in use by a thread.  Until now, ovs-vswitchd
has not waited for RCU callbacks to fire before exiting.  This meant that
in many cases, when ovs-vswitchd exits, many blocks of memory are stuck in
RCU callback queues, which valgrind often reports as "possible" memory
leaks.

This commit adds a new function ovsrcu_exit() that waits and fires as many
RCU callbacks as it reasonably can.  It can only do so for the thread that
calls it and the thread that calls the callbacks, but generally speaking
ovs-vswitchd shuts down other threads before it exits anyway, so this is
pretty good.

In my testing this eliminates most valgrind warnings for tests that run
ovs-vswitchd.  This ought to make it easier to distinguish new leaks that
are real from existing non-leaks.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmai.com>