LU-8137 utils: fix llverdev for use on regular files

LU-8137 utils: fix llverdev for use on regular files

Allow llverdev to work with regular files to verify data contents.

Add a parameter to allow specifying the maximum file size to use.
Allow the use of unit suffix when specifying sizes as arguments.
If the file size is zero, write until the filesystem is full.
Use pread() and pwrite() to avoid having to deal with file offsets.

Add sanityn test_56a/56b to verify proper operation on a large file.

Update llverdev.8 man page to describe options properly.

Test-Parameters: trivial testlist=sanityn env=ONLY=56a,56b ostsizegb=12
Test-Parameters: fstype=zfs testlist=sanityn env=ONLY=56a,56b ostsizegb=12
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ib06dc989fe692fd6aae388a79d9aa28f702540e5
Reviewed-on: https://review.whamcloud.com/43230
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Anjus George <georgea@ornl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>