cgroup: systemctl: Don't display NULL if protection was set to max

Inside format_bytes, we return NULL if the value is UINT64_MAX. This
makes some kind of sense where this has some other semantic meaning than
being a value, but in this case the value is both a.) not the default
(so we definitely want to display it), and b.) means "infinity" (or
"max" in cgroup terminology).

This patch adds a small wrapper around format_bytes that can be used for
these cases, to avoid the following situation:

    [root@tangsanjiao ~]# cat /sys/fs/cgroup/workload.slice/memory.low
    max
    [root@tangsanjiao ~]# systemctl show workload.slice -p MemoryLow
    MemoryLow=infinity
    [root@tangsanjiao ~]# systemctl status workload.slice | grep low:
	Memory: 14.9G (low: (null))

After the patch:

    [root@tangsanjiao ~]# systemctl status workload.slice | grep low:
	Memory: 15.1G (low: infinity)

src/basic/format-util.h

@@ -5,6 +5,7 @@
 #include <net/if.h>
 #include <stdbool.h>
 
+#include "cgroup-util.h"
 #include "macro.h"
 
 assert_cc(sizeof(pid_t) == sizeof(int32_t));
@@ -71,8 +72,15 @@ typedef enum {
         FORMAT_BYTES_TRAILING_B  = 1 << 2,
 } FormatBytesFlag;
 
-#define FORMAT_BYTES_MAX 8
+#define FORMAT_BYTES_MAX 16
 char *format_bytes_full(char *buf, size_t l, uint64_t t, FormatBytesFlag flag);
 static inline char *format_bytes(char *buf, size_t l, uint64_t t) {
         return format_bytes_full(buf, l, t, FORMAT_BYTES_USE_IEC | FORMAT_BYTES_BELOW_POINT | FORMAT_BYTES_TRAILING_B);
 }
+static inline char *format_bytes_cgroup_protection(char *buf, size_t l, uint64_t t) {
+        if (t == CGROUP_LIMIT_MAX) {
+                (void) snprintf(buf, l, "%s", "infinity");
+                return buf;
+        }
+        return format_bytes(buf, l, t);
+}

src/systemctl/systemctl.c

@@ -4478,11 +4478,11 @@ static void print_status_info(
 
                         printf(" (");
                         if (i->memory_min > 0) {
-                                printf("%smin: %s", prefix, format_bytes(buf, sizeof(buf), i->memory_min));
+                                printf("%smin: %s", prefix, format_bytes_cgroup_protection(buf, sizeof(buf), i->memory_min));
                                 prefix = " ";
                         }
                         if (i->memory_low > 0) {
-                                printf("%slow: %s", prefix, format_bytes(buf, sizeof(buf), i->memory_low));
+                                printf("%slow: %s", prefix, format_bytes_cgroup_protection(buf, sizeof(buf), i->memory_low));
                                 prefix = " ";
                         }
                         if (i->memory_high != CGROUP_LIMIT_MAX) {