selftests/bpf: Add a call_srcu() re-entry reproducer

Re-enter call_srcu() from a BPF program to exercise its any-context
safety, via call_rcu_tasks_trace(), which is call_srcu() on
rcu_tasks_trace_srcu_struct.

An fentry program on rcu_segcblist_enqueue() fires mid-enqueue: that
function is reached from srcu_gp_start_if_needed() with the srcu_data
->lock held.  The program does a task-storage delete, whose only deferred
work is call_rcu_tasks_trace(), re-entering the enqueue on the same CPU.
The handler matches on TID and fires once; pinning the thread removes the
migration window between picking the srcu_data and taking its lock.

Without the fix the nested call re-takes the same sdp lock and
self-deadlocks; with it the nested __call_srcu() sees interrupts disabled
and defers via irq_work, so the delete returns and the test passes.

The test skips where it does not apply: Tiny RCU has no
rcu_segcblist_enqueue() to attach to, and a UP+PREEMPT kernel pairs Tree
RCU with Tiny SRCU, so the attach succeeds but call_srcu() never reaches
the enqueue.  Tiny SRCU is told apart by srcu_expedite_current(), which it
stubs out, so on Tree SRCU a zero hit count fails rather than skips and the
reproducer cannot quietly stop reproducing.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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