i915/gem_ctx_persistence: Apply an rcu-barrier for fput cleanup

After any process termination, use an rcu-barrier to be sure that any
deferred struct file cleanup has been performed. By being consistent in
our paranoia here means that we can rule out more false positives and so
focus on what remains.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>

tests/i915/gem_ctx_persistence.c

@@ -468,6 +468,7 @@ static void test_process(int i915)
 	}
 	close(sv[0]);
 	igt_waitchildren();
+	rcu_barrier(i915); /* force the delayed fput() */
 
 	fence = recvfd(sv[1]);
 	close(sv[1]);
@@ -516,6 +517,7 @@ static void test_process_mixed(int i915, unsigned int engine)
 	}
 	close(sv[0]);
 	igt_waitchildren();
+	rcu_barrier(i915); /* force the delayed fput() */
 
 	fence[0] = recvfd(sv[1]);
 	fence[1] = recvfd(sv[1]);