diskspd sequential should allow run spec in units of total file/target size

[dl2n]

DISKSPD cannot be run in a mode which analyzes the costs/behavior of a first (second, third, etc.) loop of sequential IO through a file. For instance, filesystems have behavior which usually only occurs on the first pass like allocation and marking of valid data. This capability would also be useful as a simple seasoning method.

In this mode, instead of specifying run duration we'd instead specify the number of loops each thread should make through the target (or portion of target specified by the begin/end paramters).