overlay: Do not lose security.capability xattr over metadata only file copy-up

overlay: Do not lose security.capability xattr over metadata only file copy-up

Extend test 064 to check security.capability xattr is not lost over
copy-up of a metadata only file. This requires mounting overlay with
option metacopy=on and first trigger metadata only copy-up and then
trigger data copy-up.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>