ovl: allow zero size xattr

ovl: allow zero size xattr

Orabug: 26546379, 26540706
CVE-2016-1575
CVE-2016-1576

When ovl_copy_xattr() encountered a zero size xattr no more xattrs were
copied and the function returned success.  This is clearly not the desired
behavior.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: <stable@vger.kernel.org>
(cherry picked from commit 97daf8b97ad6f913a34c82515be64dc9ac08d63e)
Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>