selftests/bpf: Test trampoline stack args in a program that can tail call

Test the previous patch with a target program made of two call chains:

	run() -> call_stack() -> callee_stack(a, b, c, d, e, f, g, h)
	run() -> subprog_tail() -> bpf_tail_call()

callee_stack() takes eight u64 arguments, so g and h are passed on the
stack. Because run() can reach a tail call, a trampoline on any function
of the program keeps the tail call counter pointer. callee_stack()
itself does not lead to the tail call, which is why the verifier still
lets it take stack arguments.

Attach fentry and fexit programs to callee_stack(). The fentry program
checks that it sees g and h, and the test checks the value that
callee_stack() returns when the fexit trampoline calls it.

Without the previous patch, the function takes the counter pointer for
its first stack argument:

  test_stack_arg_tailcall:FAIL:stack ret unexpected stack ret: actual -359789677662885869 != expected 204
  test_stack_arg_tailcall:FAIL:stack fexit ret unexpected stack fexit ret: actual -359789677662885869 != expected 204

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261016183049.1133231-1-yonghong.song@linux.dev

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