lightnvm: potential underflow in pblk_read_rq()

This is a static checker fix, and perhaps not a real bug.  The static
checker thinks that nr_secs could be negative.  It would result in
zeroing more memory than intended.  Anyway, even if it's not a bug,
changing this variable to unsigned makes the code easier to audit.

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>