runc exec: fix setting process.Scheduler

Commit 770728e1 added Scheduler field into both Config and Process,
but forgot to add a mechanism to actually use Process.Scheduler.
As a result, runc exec does not set Process.Scheduler ever.

Fix it, and a test case (which fails before the fix).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/container_linux.go

@@ -708,6 +708,7 @@ func (c *Container) newInitConfig(process *Process) *initConfig {
 		ProcessLabel:     c.config.ProcessLabel,
 		Rlimits:          c.config.Rlimits,
 		IOPriority:       c.config.IOPriority,
+		Scheduler:        c.config.Scheduler,
 		CreateConsole:    process.ConsoleSocket != nil,
 		ConsoleWidth:     process.ConsoleWidth,
 		ConsoleHeight:    process.ConsoleHeight,
@@ -733,6 +734,9 @@ func (c *Container) newInitConfig(process *Process) *initConfig {
 	if process.IOPriority != nil {
 		cfg.IOPriority = process.IOPriority
 	}
+	if process.Scheduler != nil {
+		cfg.Scheduler = process.Scheduler
+	}
 
 	// Set misc properties.