arm: Fix draining of the pagetable walker when squashing

Since the page table walker only checks if a drain has completed in
doL1DescriptorWrapper() and doL2DescriptorWrapper(), it sometimes
looses track of a drain request if there is a squash. This changeset
adds a completeDrain() call after squashing requests in the pending
queue, which fixes this issue.

src/arch/arm/table_walker.cc

@@ -225,6 +225,7 @@ TableWalker::processWalkWrapper()
     // if we've still got pending translations schedule more work
     nextWalk(tc);
     currState = NULL;
+    completeDrain();
 }
 
 Fault