LU-11375 mdc: use old statfs format

LU-11375 mdc: use old statfs format

when the client talks to old server with no support
for aggregated statfs

Test-Parameters: clientjob=lustre-b2_10 clientbuildno=136 testgroup=review-ldiskfs

Change-Id: I447b312d46db56da152f62835b3f98401f997cf0
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33162
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Tested-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: James Simmons <uja.ornl@yahoo.com>