cgroup-util: helper to cg_get_attribute and convert to uint64_t

A common pattern in the codebase is reading a cgroup memory value
and converting it to a uint64_t. Let's make it a helper and refactor a
few places to use it so it's more concise.

src/basic/cgroup-util.c

@@ -1639,6 +1639,32 @@ int cg_get_attribute(const char *controller, const char *path, const char *attri
         return read_one_line_file(p, ret);
 }
 
+int cg_get_attribute_as_uint64(const char *controller, const char *path, const char *attribute, uint64_t *ret) {
+        _cleanup_free_ char *value = NULL;
+        uint64_t v;
+        int r;
+
+        assert(ret);
+
+        r = cg_get_attribute(controller, path, attribute, &value);
+        if (r == -ENOENT)
+                return -ENODATA;
+        if (r < 0)
+                return r;
+
+        if (streq(value, "max")) {
+                *ret = CGROUP_LIMIT_MAX;
+                return 0;
+        }
+
+        r = safe_atou64(value, &v);
+        if (r < 0)
+                return r;
+
+        *ret = v;
+        return 0;
+}
+
 int cg_get_keyed_attribute(
                 const char *controller,
                 const char *path,

src/basic/cgroup-util.h

@@ -184,6 +184,8 @@ int cg_set_attribute(const char *controller, const char *path, const char *attri
 int cg_get_attribute(const char *controller, const char *path, const char *attribute, char **ret);
 int cg_get_keyed_attribute(const char *controller, const char *path, const char *attribute, char **keys, char **values);
 
+int cg_get_attribute_as_uint64(const char *controller, const char *path, const char *attribute, uint64_t *ret);
+
 int cg_set_access(const char *controller, const char *path, uid_t uid, gid_t gid);
 
 int cg_set_xattr(const char *controller, const char *path, const char *name, const void *value, size_t size, int flags);

src/basic/limits-util.c

@@ -120,16 +120,9 @@ uint64_t system_tasks_max(void) {
         if (r < 0)
                 log_debug_errno(r, "Failed to determine cgroup root path, ignoring: %m");
         else {
-                _cleanup_free_ char *value = NULL;
-
-                r = cg_get_attribute("pids", root, "pids.max", &value);
+                r = cg_get_attribute_as_uint64("pids", root, "pids.max", &b);
                 if (r < 0)
                         log_debug_errno(r, "Failed to read pids.max attribute of cgroup root, ignoring: %m");
-                else if (!streq(value, "max")) {
-                        r = safe_atou64(value, &b);
-                        if (r < 0)
-                                log_debug_errno(r, "Failed to parse pids.max attribute of cgroup root, ignoring: %m");
-                }
         }
 
         return MIN3(TASKS_MAX,