Init buflen for our dummy buffer

Init buflen for our dummy buffer

If we fail to verify our buffer we will allocate a dummy buf to fill in so we
have something to compare our bad file to.  Unfortunately we don't set the
buflen of the dummy buffer which means we will sometimes write well past the end
of our buffer and this make glibc complain loudly.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>