unit-file: do not allow bogus IOSchedulingClass values

We have only three bits of space, i.e. 8 possible classes. Immediately reject
anything outside of that range. Add the fuzzer test case and an additional
unit test.

oss-fuzz #6908.

src/basic/process-util.c

@@ -1466,7 +1466,7 @@ static const char *const ioprio_class_table[] = {
         [IOPRIO_CLASS_IDLE] = "idle"
 };
 
-DEFINE_STRING_TABLE_LOOKUP_WITH_FALLBACK(ioprio_class, int, INT_MAX);
+DEFINE_STRING_TABLE_LOOKUP_WITH_FALLBACK(ioprio_class, int, IOPRIO_N_CLASSES);
 
 static const char *const sigchld_code_table[] = {
         [CLD_EXITED] = "exited",