ovl: fix get_acl() on tmpfs

ovl: fix get_acl() on tmpfs

tmpfs doesn't have ->get_acl() because it only uses cached acls.

This fixes the acl tests in pjdfstest when tmpfs is used as the upper layer
of the overlay.

Reported-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 39a25b2b3762 ("ovl: define ->get_acl() for overlay inodes")
Cc: <stable@vger.kernel.org> # v4.8
Orabug: 26975443

(backport upstream commit b93d4a0eb308d4400b84c8b24c1b80e09a9497d0)

Signed-off-by: Shan Hai <shan.hai@oracle.com>