Add an option to set BlockSize

[Zero3K]

It would be nice if that could be added in order to see if the current size being used is causing errors while testing a UDF drive with an open source driver being used to mount it.

[ncw]

The block size is set here

stressdisk/stressdisk.go

Line 50 in daaa039

BlockSize = 2 * MB

It would be quite easy to make it controllable by a flag if you wanted to have a go at that, eg put this just below

blockSize      = flag.Int("s", 2*MB, "Size of the read / write buffer")

And replace all occurrences of BlockSize with *blockSize

[Zero3K]

Actually, this is not needed since I was able to figure out the reason why the driver is causing errors when using StressDisk. It has a bug(s) with the use of DirectIO.