selftests/bpf: Fix compile error from rlim_t in sk_storage_map.c

Cast 'rlim_t' argument to match expected type of printf() format and avoid
compile errors seen building for mips64el/musl-libc:

  In file included from map_tests/sk_storage_map.c:20:
  map_tests/sk_storage_map.c: In function 'test_sk_storage_map_stress_free':
  map_tests/sk_storage_map.c:414:56: error: format '%lu' expects argument of type 'long unsigned int', but argument 2 has type 'rlim_t' {aka 'long long unsigned int'} [-Werror=format=]
    414 |                 CHECK(err, "setrlimit(RLIMIT_NOFILE)", "rlim_new:%lu errno:%d",
        |                                                        ^~~~~~~~~~~~~~~~~~~~~~~
    415 |                       rlim_new.rlim_cur, errno);
        |                       ~~~~~~~~~~~~~~~~~
        |                               |
        |                               rlim_t {aka long long unsigned int}
  ./test_maps.h:12:24: note: in definition of macro 'CHECK'
     12 |                 printf(format);                                         \
        |                        ^~~~~~
  map_tests/sk_storage_map.c:414:68: note: format string is defined here
    414 |                 CHECK(err, "setrlimit(RLIMIT_NOFILE)", "rlim_new:%lu errno:%d",
        |                                                                  ~~^
        |                                                                    |
        |                                                                    long unsigned int
        |                                                                  %llu
  cc1: all warnings being treated as errors

Fixes: 51a0e301 ("bpf: Add BPF_MAP_TYPE_SK_STORAGE test to test_maps")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1e00a1fa7acf91b4ca135c4102dc796d518bad86.1721713597.git.tony.ambardar@gmail.com

tools/testing/selftests/bpf/map_tests/sk_storage_map.c

@@ -412,7 +412,7 @@ static void test_sk_storage_map_stress_free(void)
 		rlim_new.rlim_max = rlim_new.rlim_cur + 128;
 		err = setrlimit(RLIMIT_NOFILE, &rlim_new);
 		CHECK(err, "setrlimit(RLIMIT_NOFILE)", "rlim_new:%lu errno:%d",
-		      rlim_new.rlim_cur, errno);
+		      (unsigned long) rlim_new.rlim_cur, errno);
 	}
 
 	err = do_sk_storage_map_stress_free();