blktap: Fix compile errors

When compiling the various blktap drivers with warnings cranked up, I
get errors that return values are not checked.

Signed-off-by: Charles Coffing <ccoffing@novell.com>

tools/blktap/drivers/blktapctrl.c

@@ -678,7 +678,10 @@ int main(int argc, char *argv[])
 	__init_blkif();
 	snprintf(buf, sizeof(buf), "BLKTAPCTRL[%d]", getpid());
 	openlog(buf, LOG_CONS|LOG_ODELAY, LOG_DAEMON);
-	daemon(0,0);
+	if (daemon(0,0)) {
+		DPRINTF("daemon failed (%d)\n", errno);
+		goto open_failed;
+	}
 
 	print_drivers();
 	init_driver_list();

tools/blktap/drivers/block-qcow.c

@@ -745,7 +745,10 @@ found:
 		}
 		memcpy(tmp_ptr2, l2_ptr, 4096);
 		lseek(s->fd, l2_offset + (l2_sector << 12), SEEK_SET);
-		write(s->fd, tmp_ptr2, 4096);
+		if (write(s->fd, tmp_ptr2, 4096) != 4096) {
+			free(tmp_ptr2);
+			return -1;
+		}
 		free(tmp_ptr2);
 	}
 	return cluster_offset;
@@ -1252,8 +1255,8 @@ int qcow_create(const char *filename, uint64_t total_size,
 				strncpy(backing_filename, backing_file,
 					sizeof(backing_filename));
 			} else {
-				realpath(backing_file, backing_filename);
-				if (stat(backing_filename, &st) != 0) {
+				if (realpath(backing_file, backing_filename) == NULL ||
+				    stat(backing_filename, &st) != 0) {
 					return -1;
 				}
 			}

tools/blktap/drivers/block-vmdk.c

@@ -283,8 +283,9 @@ static uint64_t get_cluster_offset(struct tdvmdk_state *prv,
         	if (!allocate)
             		return 0;
         	cluster_offset = lseek(prv->fd, 0, SEEK_END);
-        	ftruncate(prv->fd, cluster_offset + 
-			  (prv->cluster_sectors << 9));
+        	if (ftruncate(prv->fd, cluster_offset + 
+			      (prv->cluster_sectors << 9)))
+			return 0;
         	cluster_offset >>= 9;
         	/* update L2 table */
         	tmp = cpu_to_le32(cluster_offset);