EX-8349 lipe: Fix problem with fallocate in test_306

EX-8349 lipe: Fix problem with fallocate in test_306

This patch modifies the fallocate command so that the
size type is specified correctly on all versions.
This should will fix the issue with invalid
length value.

Test-Parameters: trivial
Signed-off-by: Vitaliy Kuznetsov <vkuznetsov@ddn.com>
Change-Id: Ie25dc87aed8ea9c882d9b352b11a58071c9c460d
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52779
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>