ovl: use a minimal buffer in ovl_copy_xattr

ovl: use a minimal buffer in ovl_copy_xattr

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

Rather than always allocating the high-order XATTR_SIZE_MAX buffer
which is costly and prone to failure, only allocate what is needed and
realloc if necessary.

Fixes https://github.com/coreos/bugs/issues/489

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Cc: <stable@vger.kernel.org>
(cherry picked from commit e4ad29fa0d224d05e08b2858e65f112fd8edd4fe)
Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>