write back cpu.max.burst after cpu.max modified in cgroup v2

If the burst is not set, we need to read the current
value to write it back after `cpu.max` modified.

Signed-off-by: lifubang <lifubang@acmcoder.com>

libcontainer/cgroups/fs2/cpu.go

@@ -51,6 +51,15 @@ func setCpu(dirPath string, r *configs.Resources) error {
 		} else {
 			burst = ""
 		}
+	} else {
+		// If the burst is not set, we need to read the current
+		// value to write it back after `cpu.max` modified.
+		if r.CpuQuota != 0 || r.CpuPeriod != 0 {
+			burst, _ = cgroups.ReadFile(dirPath, "cpu.max.burst")
+			if burst == "0" {
+				burst = ""
+			}
+		}
 	}
 	if r.CpuQuota != 0 || r.CpuPeriod != 0 {
 		str := "max"