ovl: remove privs in ovl_copyfile()

Underlying fs doesn't remove privs because copy_range/remap_range are
called with privileged mounter credentials.

This fixes some failures in fstest generic/673.

Fixes: 8ede2055 ("ovl: add reflink/copyfile/dedup support")
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>