Add possibility to make sequential IO "holed"

Add possibility to make sequential IO "holed"

For sequential IO, it is now possible to add a number of bytes to
be skipped for every block read or written. Using:

bs=8k
rw=read:8k

will first read 0k->8k, then 16k->24k, and so on. This skips 8k
for every 'bs' sized block read. Similar for writes, doing

bs=4k
rw=write:4k

will write 0k->4k, then 8k->12k, etc. End result being that every
other block is written, in a sequential fashion.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>