overlay: copy-up lowerdir which has opaque xattr set

overlay: copy-up lowerdir which has opaque xattr set

When copying up a directory with opaque xattr set, the xattr was
copied up as well, after a remount the opaque flag is honored so the
copied-up directory is treated as opaque, which causes files created
in the lowerdir to be hidden when using multiple lower directories.

This is fixed by "ovl: don't copy up opaqueness".

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>