ovl: remove privs in ovl_fallocate()

Underlying fs doesn't remove privs because fallocate is called with
privileged mounter credentials.

This fixes some failure in fstests generic/683..687.

Fixes: aab8848c

 ("ovl: add ovl_fallocate()")
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>