Fix problem when update memory and swap memory

Currently, if we start a container with:
`docker run -ti --name foo --memory 300M --memory-swap 500M busybox sh`

Then we want to update it with:
`docker update --memory 600M --memory-swap 800M foo`

It'll get error because we can't set memory to 600M with
the 500M limit of swap memory.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

libcontainer/cgroups/fs/memory.go

@@ -66,13 +66,20 @@ func (s *MemoryGroup) SetKernelMemory(path string, cgroup *configs.Cgroup) error
 }
 
 func (s *MemoryGroup) Set(path string, cgroup *configs.Cgroup) error {
-	if cgroup.Resources.Memory != 0 {
-		if err := writeFile(path, "memory.limit_in_bytes", strconv.FormatInt(cgroup.Resources.Memory, 10)); err != nil {
+	// When memory and swap memory are both set, we need to release the limits first,
+	// it's to avoid problem when we update the configs, the new memory limit could
+	// be larger than the existed swap memory limit but smaller than the new swap
+	// memory limit, we should allow such update usage.
+	if cgroup.Resources.Memory != 0 && cgroup.Resources.MemorySwap != 0 {
+		if err := writeFile(path, "memory.memsw.limit_in_bytes", "-1"); err != nil {
+			return err
+		}
+		if err := writeFile(path, "memory.limit_in_bytes", "-1"); err != nil {
 			return err
 		}
 	}
-	if cgroup.Resources.MemoryReservation != 0 {
-		if err := writeFile(path, "memory.soft_limit_in_bytes", strconv.FormatInt(cgroup.Resources.MemoryReservation, 10)); err != nil {
+	if cgroup.Resources.Memory != 0 {
+		if err := writeFile(path, "memory.limit_in_bytes", strconv.FormatInt(cgroup.Resources.Memory, 10)); err != nil {
 			return err
 		}
 	}
@@ -81,6 +88,11 @@ func (s *MemoryGroup) Set(path string, cgroup *configs.Cgroup) error {
 			return err
 		}
 	}
+	if cgroup.Resources.MemoryReservation != 0 {
+		if err := writeFile(path, "memory.soft_limit_in_bytes", strconv.FormatInt(cgroup.Resources.MemoryReservation, 10)); err != nil {
+			return err
+		}
+	}
 	if cgroup.Resources.OomKillDisable {
 		if err := writeFile(path, "memory.oom_control", "1"); err != nil {
 			return err