LU-16030 pcc: enlarge PCC backend size for sanity-pcc/45

LU-16030 pcc: enlarge PCC backend size for sanity-pcc/45

sanity-pcc/test_45 failed with the following error:
"attach more than 2 time: 0"
The reason is that the allocated loopback deivce for PCC backend
is too small.
In this patch, we enlarge it to 100MiB to avoid running out of
the space on PCC backend device during PCC-RO attach and fix the
failure of sanity-pcc/test_45.

This patch also fixes the typo error "RANDON" -> "RANDOM".

Test-Parameters: trivial testlist=sanity-pcc
Change-Id: Ib19f01ed054cb6c9eecceabea1f1da72dea0b113
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/47994
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>