target: avoid accessing .bi_vcnt directly

When the bio is full, bio_add_pc_page() will return zero,
so use this information tell when the bio is full.

Also replace access to .bi_vcnt for pr_debug() with bio_segments().
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/target/target_core_pscsi.c

@@ -935,13 +935,9 @@ pscsi_map_sg(struct se_cmd *cmd, struct scatterlist *sgl, u32 sgl_nents,
 
 			rc = bio_add_pc_page(pdv->pdv_sd->request_queue,
 					bio, page, bytes, off);
-			if (rc != bytes)
-				goto fail;
-
 			pr_debug("PSCSI: bio->bi_vcnt: %d nr_vecs: %d\n",
-				bio->bi_vcnt, nr_vecs);
-
-			if (bio->bi_vcnt > nr_vecs) {
+				bio_segments(bio), nr_vecs);
+			if (rc != bytes) {
 				pr_debug("PSCSI: Reached bio->bi_vcnt max:"
 					" %d i: %d bio: %p, allocating another"
 					" bio\n", bio->bi_vcnt, i, bio);