core/mount: Only remove dirs if unmount succeeded

The detached mount is less likely to fail in our case, but if we see any
failure to unmount, we should just skip the removal of directories.

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

core/mount/mount_linux.go

@@ -255,6 +255,7 @@ func doPrepareIDMappedOverlay(lowerDirs []string, usernsFd int) (tmpLowerDirs []
 			// gone (eventually) and we can safely delete the directory too.
 			if err := unix.Unmount(lowerDir, unix.MNT_DETACH); err != nil {
 				log.L.WithError(err).Warnf("failed to unmount temp lowerdir %s", lowerDir)
+				continue
 			}
 			// Using os.Remove() so if it's not empty, we don't delete files in the
 			// rootfs.