ovl: use a minimal buffer in ovl_copy_xattr

commit e4ad29fa0d224d05e08b2858e65f112fd8edd4fe upstream.

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 coreos/bugs#489

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>