selftests/livepatch: better synchronize test_klp_callbacks_busy

The test_klp_callbacks_busy module conditionally blocks a future
livepatch transition by busy waiting inside its workqueue function,
busymod_work_func().  After scheduling this work, a test livepatch is
loaded, introducing the transition under test.

Both events are marked in the kernel log for later verification, but
there is no synchronization to ensure that busymod_work_func() logs its
function entry message before subsequent selftest commands log their own
messages.  This can lead to a rare test failure due to unexpected
ordering like:

  --- expected
  +++ result
  @@ -1,7 +1,7 @@
   % modprobe test_klp_callbacks_busy block_transition=Y
   test_klp_callbacks_busy: test_klp_callbacks_busy_init
  -test_klp_callbacks_busy: busymod_work_func enter
   % modprobe test_klp_callbacks_demo
  +test_klp_callbacks_busy: busymod_work_func enter
   livepatch: enabling patch 'test_klp_callbacks_demo'
   livepatch: 'test_klp_callbacks_demo': initializin...