LU-11268 mdc: use smaller buffers for getxattr

LU-11268 mdc: use smaller buffers for getxattr

In mdc_intent_getxattr_pack() only the xattr values buffer should be
upsized for older MDTs. This refines the interop fix from change
https://review.whamcloud.com/31990 (LU-10912 mdc: use large xattr
buffers for old servers).

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: If7c6fe7be96a6a227c746f90757b7aae1874b14e
Reviewed-on: https://review.whamcloud.com/33024
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>