Don't populate random data for verify reads

Don't populate random data for verify reads

I was seeing some performance drop during the read verification phase
of a test, and from the code in io_u.c in get_io_u function, we
prepare/populate the buffer in the io_u structure based on the verify
patterns/options.
This makes sense when we are doing writes, but I dont understand why
we do this for the read phase when this data is going to be
overwritten anyways(and in case of truncated reads, we do modify the
buf_len).
So based on that I changed the code to populate the buffer(io_u->buf)
only if its a write with verify enabled.
This works for my tests, but I do not know if there was a reason why
this was populated for reads as well to begin with.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>