selftests/bpf: fix task_local_storage/exit_creds rcu usage

BPF CI has revealed flakiness in the task_local_storage/exit_creds test.
The failure point in CI [1] is that null_ptr_count is equal to 0,
which indicates that the program hasn't run yet. This points to the
kern_sync_rcu (sys_membarrier -> synchronize_rcu underneath) not
waiting sufficiently.

Indeed, synchronize_rcu only waits for read-side sections that started
before the call. If the program execution starts *during* the
synchronize_rcu invocation (due to, say, preemption), the test won't
wait long enough.

As a speculative fix, make the synchornize_rcu calls in a loop until
an explicit run counter has gone up.

  [1]: https://github.com/kernel-patches/bpf/actions/runs/3268263235/jobs/5374940791

Signed-off-by: Delyan Kratunov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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