selftests/bpf: Add test for stack arg read without caller write

selftests/bpf: Add test for stack arg read without caller write

Add negative tests for the outgoing stack arg validation.
A static subprog with a 'long *' arg causes
btf_prepare_func_args() to fail after setting arg_cnt. The
validation ensures check_outgoing_stack_args() still runs.

Also update two existing tests (release_ref, stale_pkt_ptr) whose
expected error messages changed: invalidated stack arg slots are now
caught by check_outgoing_stack_args() at the call site instead of
at the callee's dereference.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260515225045.822104-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>