LU-11873 tests: Increase barrier freeze time

LU-11873 tests: Increase barrier freeze time

Barrier freeze times of 10 seconds or less are roughly the
same length as ZFS commit intervals, and because barriers
generate sync ops, they have to wait for those.  This means
that a 10 second barrier will occassionally expire before
the commit has finished.

Switch to barriers of at least 20 seconds.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I50fc8315c791ed444ccf39755441fdbe3aa1db6c
Reviewed-on: https://review.whamcloud.com/35361
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>