ublk: fix build warning on iov_iter_get_pages2

Return type of iov_iter_get_pages2() is ssize_t instead of size_t, so
fix it.

Fixes: 981f95a571 ("ublk: cleanup ublk_copy_user_pages")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230520151134.459679-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -564,7 +564,8 @@ static size_t ublk_copy_user_pages(const struct request *req,
 
 	while (iov_iter_count(uiter) && iter.bio) {
 		unsigned nr_pages;
-		size_t len, off;
+		ssize_t len;
+		size_t off;
 		int i;
 
 		len = iov_iter_get_pages2(uiter, iter.pages,