selftests/bpf: Test one may_goto time slice per prog invocation

Most tests call a function that spins until may_goto stops it, four times.
With one time slice for an invocation of the prog only the first call
spins. With a time slice for each frame all four calls spin, and the test
fails after 1 sec.

  group           tests
  --------------  ------------------------------------------------------
  the problem     9 insns, three levels of short loops, a subprog with
                  65534 iterations, bpf_loop() with a callback that spins
                  or makes 60000 iterations
  ways to a loop  call, call through a function without a loop, global
                  function, callx, bpf_loop() inlined and not,
                  bpf_for_each_map_elem(), bpf_user_ringbuf_drain(),
                  bpf_find_vma(), bpf_rbtree_add()
  bpf_loop()      ends when the time is up: inlined and not, in a callback
                  of bpf_loop(). Goes on when BPF code calls the callback
                  too
  count           a loop that is on the stack ends when the time is up,
                  65535 calls that make one iteration read the clock
  new time slice  timer, wq, task_work, call_rcu, call_rcu_tasks_trace,
                  each after a prog on the cpu that used up its time. Tail
                  call, extension prog
  same one        a timer callback that interrupts the prog doesn't give it
                  more time, the exception callback
  registers       may_goto takes a register that is dead: R0-RN are live,
                  for each N. A register that is live through the back
                  edge only, at the target only, behind the may_goto only.
                  All of that when it calls C and when the time is up.
                  16-byte return value
  other           tracing prog with frames in a private stack, may_goto in
                  dead code

Two tests lock up a cpu on a kernel with a time slice for each frame: the
9 insns and the three levels of short loops. All other tests give up and
fail there, or pass. The tests are for x86-64 and arm64 with the JIT only.

may_goto_far gets four subtests with the may_goto in a subprog.

The test is serial. Its progs spin for 250 ms in an RCU read-side critical
section, one after the other. Next to them in 'test_progs -j' the test
"mmap" gives up its wait for the end of a map. The test takes 10 sec.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261031215543.682284-10-alexei.starovoitov@gmail.com

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