Only warn on OOMKill disable if the feature is actually requested

Instead of warning the caller who is disabling OOM killer that the
feature isn't available, only warn if they are trying to **enable** OOM
killer and it can't be done.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

daemon/daemon_unix.go

@@ -275,8 +275,12 @@ func verifyContainerResources(resources *containertypes.Resources) ([]string, er
 		logrus.Warnf("You specified a kernel memory limit on a kernel older than 4.0. Kernel memory limits are experimental on older kernels, it won't work as expected and can cause your system to be unstable.")
 	}
 	if resources.OomKillDisable != nil && !sysInfo.OomKillDisable {
-		warnings = append(warnings, "Your kernel does not support OomKillDisable, OomKillDisable discarded.")
-		logrus.Warnf("Your kernel does not support OomKillDisable, OomKillDisable discarded.")
+		// only produce warnings if the setting wasn't to *disable* the OOM Kill; no point
+		// warning the caller if they already wanted the feature to be off
+		if *resources.OomKillDisable {
+			warnings = append(warnings, "Your kernel does not support OomKillDisable, OomKillDisable discarded.")
+			logrus.Warnf("Your kernel does not support OomKillDisable, OomKillDisable discarded.")
+		}
 		resources.OomKillDisable = nil
 	}