LU-1823 mdd: Don't provide buffer for xattr when we just need size

LU-1823 mdd: Don't provide buffer for xattr when we just need size

mdd_declare_attr_set incorrectly passed uninitialized buffer to mdo_xattr_get
where as it is just interested in knowing if xattr exists at all.
The buffer happens to point to other people's data sometimes which leads
to random memory corruptions.

Change-Id: I28b477d385427292ad38455d495ab546f757c7d5
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/3928
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>