selftests/bpf: Add test case for the freeing of bpf_timer

The main purpose of the test is to demonstrate the lock problem for the
free of bpf_timer under PREEMPT_RT. When freeing a bpf_timer which is
running on other CPU in bpf_timer_cancel_and_free(), hrtimer_cancel()
will try to acquire a spin-lock (namely softirq_expiry_lock), however
the freeing procedure has already held a raw-spin-lock.

The test first creates two threads: one to start timers and the other to
free timers. The start-timers thread will start the timer and then wake
up the free-timers thread to free these timers when the starts complete.
After freeing, the free-timer thread will wake up the start-timer thread
to complete the current iteration. A loop of 10 iterations is used.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20250117101816.2101857-6-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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