daemon/graphdriver/overlay2: remove deprecated overrideKernelCheck

Commit 955c1f881ac94af19c99f0f7d5635e6a574789f2 (Docker v17.12.0) replaced
detection of support for multiple lowerdirs (as required by overlay2) to not
depend on the kernel version. The `overlay2.override_kernel_check` was still
used to print a warning that older kernel versions may not have full support.

After this, commit e226aea280efdc18f887e853b5eefd763b602937 (Docker v20.10.0,
backported to v19.03.7) removed uses of the `overlay2.override_kernel_check`
option altogether, but we were still parsing it.

This patch changes the `parseOptions()` function to not parse the option,
printing a deprecation warning instead. We should change this to be an error,
but the  `overlay2.override_kernel_check` option was not deprecated in the
documentation, so keeping it around for one more release.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/overlay2/overlay.go

@@ -85,8 +85,7 @@ const (
 )
 
 type overlayOptions struct {
-	overrideKernelCheck bool
-	quota               quota.Quota
+	quota quota.Quota
 }
 
 // Driver contains information about the home directory and the list of active
@@ -236,10 +235,8 @@ func parseOptions(options []string) (*overlayOptions, error) {
 		key = strings.ToLower(key)
 		switch key {
 		case "overlay2.override_kernel_check":
-			o.overrideKernelCheck, err = strconv.ParseBool(val)
-			if err != nil {
-				return nil, err
-			}
+			// TODO(thaJeztah): change this to an error, see https://github.com/docker/cli/pull/3806
+			logger.Warn("DEPRECATED: the overlay2.override_kernel_check option is ignored and will be removed in the next release. You can safely remove this option from your configuration.")
 		case "overlay2.size":
 			size, err := units.RAMInBytes(val)
 			if err != nil {