selftests/bpf: Add tests for rqspinlock

Introduce selftests that trigger AA, ABBA deadlocks, and test the edge
case where the held locks table runs out of entries, since we then
fallback to the timeout as the final line of defense. Also exercise
verifier's AA detection where applicable.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250316040541.108729-26-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>