selftests/bpf: Check stack_mprotect() return value

If stack_mprotect() succeeds, errno is not changed. This can produce
misleading error messages, that show stale errno.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-13-iii@linux.ibm.com


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