EX-8349 lipe: Fix problem with fallocate in test_306

EX-8349 lipe: Fix problem with fallocate in test_306

This patch removes the check every time a file is created,
and adds a general check for the presence of created files.

Test-Parameters: trivial testlist=sanity-lipe-scan3
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: I7d7e0c9b0e6517853bb3b77e8f938bbd877b3003
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52590
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>