set_lsb_release.sh: only setfattr for selinux if modified

For set_lsb_release.sh called without parameter, it doesn't modify
anything in the image, and mount the image ro. Thus setfattr to
ensure security.selinux xattr will fail with Read-only filesystem,
and is not necessary since nothing has been modified.

BUG=chromium:954670
TEST=set_lsb_release.sh xx.bin a b
TEST=set_lsb_release.sh xx.bin
BRANCH=none

Change-Id: I32bf61796c2b60d18e4e62cc43f2d0e9dc75cef5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1666516
Tested-by: Qijiang Fan <fqj@google.com>
Commit-Queue: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
1 file changed