revalidator: Avoid assert in transition_ukey().

There is a case where a flow is dumped from the kernel after the ukey is
already transitioned into an EVICTING/EVICTED/DELETED state, and the
revalidator thread attempts to shift that into UKEY_OPERATIONAL because
it was able to dump the flow from the datapath. This resulted in
triggering the assert in transition_ukey(). Detect this condition and
skip handling the flow (as it's already on its way out).

Users report:
> Program terminated with signal SIGABRT, Aborted.
> raise () from /lib/x86_64-linux-gnu/libc.so.6
> raise () from /lib/x86_64-linux-gnu/libc.so.6
> abort () from /lib/x86_64-linux-gnu/libc.so.6
> ovs_abort_valist
> vlog_abort_valist
> vlog_abort
> ovs_assert_failure
> transition_ukey (ukey=<optimized out>, dst=<optimized out>)
>     at ofproto/ofproto-dpif-upcall.c:1674
> revalidate (revalidator=0x1cb36c8) at ofproto/ofproto-dpif-upcall.c:2324
> udpif_revalidator (arg=0x1cb36c8) at ofproto/ofproto-dpif-upcall.c:901
> ovsthread_wrapper (aux_=<optimized out>) at lib/ovs-thread.c:348
> start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
> clone () from /lib/x86_64-linux-gnu/libc.so.6

VMware-BZ: #1857694
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

ofproto/ofproto-dpif-upcall.c

@@ -2316,8 +2316,13 @@ revalidate(struct revalidator *revalidator)
                 continue;
             }
 
-            /* The flow is now confirmed to be in the datapath. */
-            transition_ukey(ukey, UKEY_OPERATIONAL);
+            if (ukey->state <= UKEY_OPERATIONAL) {
+                /* The flow is now confirmed to be in the datapath. */
+                transition_ukey(ukey, UKEY_OPERATIONAL);
+            } else {
+                ovs_mutex_unlock(&ukey->mutex);
+                continue;
+            }
 
             if (!used) {
                 used = ukey->created;