selftests/bpf: Set errno when urand_spawn() fails

The result of urand_spawn() is checked with ASSERT_OK_PTR, which treats
NULL as success if errno == 0.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-8-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/usdt.c

@@ -314,6 +314,7 @@ static FILE *urand_spawn(int *pid)
 
 	if (fscanf(f, "%d", pid) != 1) {
 		pclose(f);
+		errno = EINVAL;
 		return NULL;
 	}