selftests/bpf: Add timer lockup selftest

Add a selftest that tries to trigger a situation where two timer callbacks
are attempting to cancel each other's timer. By running them continuously,
we hit a condition where both run in parallel and cancel each other.

Without the fix in the previous patch, this would cause a lockup as
hrtimer_cancel on either side will wait for forward progress from the
callback.

Ensure that this situation leads to a EDEADLK error.

Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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