Move assert(count <= reqs_in_uring); inside critical section

We otherwise may run into the assert as another thread may already
decremented regs_in_uring.
22 jobs for completer-strategies in 6 minutes and 10 seconds (queued for 1 minute and 28 seconds)
latest
Status Name Job ID Coverage
  Smoke Test
passed clang-tidy #436697

00:01:59

passed fast-static-analysis #436695

00:01:27

passed iwyu #436696

00:01:52

passed smoke-test-suite #436694

00:01:13

 
  Test
passed test-clang #436701

00:02:16

passed test-clang-debug #436703

00:04:05

passed test-clang-release #436702

00:03:51

passed test-clang-sanitizer-undefined #436707

00:03:52

passed test-do-not-log-timestamp #436711

00:01:07

passed test-futex-wakeup-semaphore #436713

00:01:19

passed test-gcc #436698

00:02:10

passed test-gcc-debug #436700

00:03:38

passed test-gcc-release #436699

00:03:34

passed test-laws #436708

00:01:14

passed test-laws-release #436709

00:01:34

passed test-locked-wakeup-semaphore #436714

00:02:05

passed test-locked-ws-queues #436712

00:02:29

passed test-set-affinity-on-block #436715

00:00:49

passed test-with-stats #436705

00:03:43

passed test-with-userspace-rcu #436706

00:04:07

passed test-worker-no-sleep #436704

00:03:41

passed test-worker-wakeup-strategy-all #436710

00:02:14