selftests/bpf: Add tests for preempt kfuncs

selftests/bpf: Add tests for preempt kfuncs

Add tests for nested cases, nested count preservation upon different
subprog calls that disable/enable preemption, and test sleepable helper
call in non-preemptible regions.

182/1   preempt_lock/preempt_lock_missing_1:OK
182/2   preempt_lock/preempt_lock_missing_2:OK
182/3   preempt_lock/preempt_lock_missing_3:OK
182/4   preempt_lock/preempt_lock_missing_3_minus_2:OK
182/5   preempt_lock/preempt_lock_missing_1_subprog:OK
182/6   preempt_lock/preempt_lock_missing_2_subprog:OK
182/7   preempt_lock/preempt_lock_missing_2_minus_1_subprog:OK
182/8   preempt_lock/preempt_balance:OK
182/9   preempt_lock/preempt_balance_subprog_test:OK
182/10  preempt_lock/preempt_global_subprog_test:OK
182/11  preempt_lock/preempt_sleepable_helper:OK
182     preempt_lock:OK
Summary: 1/11 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20240424031315.2757363-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>