LU-13359 quota: make used for pool correct

LU-13359 quota: make used for pool correct

Before this patch used space for quota pool
was a sum of a space used by user at all OSTs
in a system. Now it is fixed and lfs quota --pool
takes into account only OSTs form the pool.
With option -v it also shows only OSTs from the pool.

Change-Id: Idf1c8ed66fca7caec70246ea4182df883bcef23c
Signed-off-by: Sergey Cheremencev <sergey.cheremencev@hpe.com>
Reviewed-on: https://review.whamcloud.com/39298
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>