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: a4bd217b43 ("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>

drivers/lightnvm/pblk-read.c

@@ -288,7 +288,7 @@ retry:
 int pblk_submit_read(struct pblk *pblk, struct bio *bio)
 {
 	struct nvm_tgt_dev *dev = pblk->dev;
-	int nr_secs = pblk_get_secs(bio);
+	unsigned int nr_secs = pblk_get_secs(bio);
 	struct nvm_rq *rqd;
 	unsigned long read_bitmap; /* Max 64 ppas per request */
 	unsigned int bio_init_idx;