LU-13296 obd: make statfs cache working again

LU-13296 obd: make statfs cache working again

Once statfs raced on mutex, lets read a cached data instead
of trash.

Test-Parameters: testlist=sanity envdefinitions=ONLY=423,ONLY_REPEAT=500
Fixes: 1c41a6ac390b ("LU-12368 obdclass: don't send multiple statfs RPCs")
Signed-off-by: Alexey Lyashkov <c17817@cray.com>
Change-Id: I268782875c30c078f239c194f69cdf7506d66169
Reviewed-on: https://review.whamcloud.com/37753
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>