nvmet: avoid integer overflow in the discard code

Although I'm not sure whether it is a good idea to support large discard
commands, I think integer overflow for discard ranges larger than 4 GB
should be avoided. This patch avoids that smatch reports the following:

drivers/nvme/target/io-cmd-file.c:249:1 nvmet_file_execute_discard() warn: should '((range.nlb)) << req->ns->blksize_shift' be a 64 bit type?

Fixes: d5eff33e ("nvmet: add simple file backed ns support")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/io-cmd-file.c

@@ -246,7 +246,8 @@ static void nvmet_file_execute_discard(struct nvmet_req *req)
 			break;
 
 		offset = le64_to_cpu(range.slba) << req->ns->blksize_shift;
-		len = le32_to_cpu(range.nlb) << req->ns->blksize_shift;
+		len = le32_to_cpu(range.nlb);
+		len <<= req->ns->blksize_shift;
 		if (offset + len > req->ns->size) {
 			ret = NVME_SC_LBA_RANGE | NVME_SC_DNR;
 			break;