selftests/bpf: Fix test_progs compilation failure in 32-bit arch

test_progs fails to be compiled in the 32-bit arch, log is as follows:

  test_progs.c:1013:52: error: format '%ld' expects argument of type 'long int', but argument 3 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
   1013 |                 sprintf(buf, "MSG_TEST_LOG (cnt: %ld, last: %d)",
        |                                                  ~~^
        |                                                    |
        |                                                    long int
        |                                                  %d
   1014 |                         strlen(msg->test_log.log_buf),
        |                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |                         |
        |                         size_t {aka unsigned int}

Fix it.

Fixes: 91b2c0af

 ("selftests/bpf: Add parallelism to test_progs")
Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221108015857.132457-1-yangjihong1@huawei.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>