m/p/blockdev: use errors.ErrUnsupported in 1.21

Change-Id: Ice8780b092c034cf755aa409ef5fce5a8aef9226
Reviewed-on: https://review.monogon.dev/c/monogon/+/2147
Tested-by: Jenkins CI
Reviewed-by: Tim Windelschmidt <tim@monogon.tech>
diff --git a/metropolis/pkg/blockdev/blockdev_linux.go b/metropolis/pkg/blockdev/blockdev_linux.go
index a8e6a8a..b40e5cd 100644
--- a/metropolis/pkg/blockdev/blockdev_linux.go
+++ b/metropolis/pkg/blockdev/blockdev_linux.go
@@ -51,7 +51,7 @@
 		return ctrlErr
 	}
 	if err == unix.EOPNOTSUPP {
-		return ErrUnsupported
+		return errors.ErrUnsupported
 	}
 	if err != unix.Errno(0) {
 		return fmt.Errorf("failed to discard: %w", err)
@@ -213,7 +213,7 @@
 		return ctrlErr
 	}
 	if errors.Is(err, unix.EOPNOTSUPP) {
-		return ErrUnsupported
+		return errors.ErrUnsupported
 	}
 	if err != unix.Errno(0) {
 		return fmt.Errorf("failed to discard: %w", err)