Fix transitions in CXQ state machine

transition() did not actually check the current state, which caused IO
handlers to open the queue prematurely, leading to crashes in a
multi-process setup.

There probably still lurks a race condition without an atomic "LOCK
CMPXCHG" operation, as commented in the code.

The CXQ state machine is called from the push()/pull() methods and
could introduce branches that might have a negative impact on
performance (side traces).

src/apps/mellanox/connectx4.lua

@@ -163,8 +163,12 @@ local DEAD = 3
 -- Returns true on successful transition, false if oldstate does not match.
 function transition (cxq, oldstate, newstate)
    -- XXX use atomic x86 "LOCK CMPXCHG" instruction. Have to teach DynASM.
-   cxq.state = newstate
-   return true
+   if cxq.state == oldstate then
+      cxq.state = newstate
+      return true
+   else
+      return false
+   end
 end
 
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