fix: linter issue

megacheck, gosimple and unused has been deprecated and subsumed by
staticcheck. And staticcheck also has been upgraded. we need to update
code for the linter issue.

close: #2945

Signed-off-by: Wei Fu <fuweid89@gmail.com>

diff/apply/apply.go

@@ -58,7 +58,7 @@ func (s *fsApplier) Apply(ctx context.Context, desc ocispec.Descriptor, mounts [
 	defer func() {
 		if err == nil {
 			log.G(ctx).WithFields(logrus.Fields{
-				"d":     time.Now().Sub(t1),
+				"d":     time.Since(t1),
 				"dgst":  desc.Digest,
 				"size":  desc.Size,
 				"media": desc.MediaType,

diff/lcow/lcow.go

@@ -99,7 +99,7 @@ func (s windowsLcowDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mou
 	defer func() {
 		if err == nil {
 			log.G(ctx).WithFields(logrus.Fields{
-				"d":     time.Now().Sub(t1),
+				"d":     time.Since(t1),
 				"dgst":  desc.Digest,
 				"size":  desc.Size,
 				"media": desc.MediaType,

diff/windows/windows.go

@@ -92,7 +92,7 @@ func (s windowsDiff) Apply(ctx context.Context, desc ocispec.Descriptor, mounts
 	defer func() {
 		if err == nil {
 			log.G(ctx).WithFields(logrus.Fields{
-				"d":     time.Now().Sub(t1),
+				"d":     time.Since(t1),
 				"dgst":  desc.Digest,
 				"size":  desc.Size,
 				"media": desc.MediaType,