block: make add_partition() return pointer to hd_struct

Make add_partition() return pointer to the new hd_struct on success
and ERR_PTR() value on failure.  This change will be used to fix md
autodetection bug.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/ioctl.c b/block/ioctl.c
index c832d63..d03985b 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -18,7 +18,6 @@
 	struct disk_part_iter piter;
 	long long start, length;
 	int partno;
-	int err;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EACCES;
@@ -61,10 +60,10 @@
 			disk_part_iter_exit(&piter);
 
 			/* all seems OK */
-			err = add_partition(disk, partno, start, length,
-					    ADDPART_FLAG_NONE);
+			part = add_partition(disk, partno, start, length,
+					     ADDPART_FLAG_NONE);
 			mutex_unlock(&bdev->bd_mutex);
-			return err;
+			return IS_ERR(part) ? PTR_ERR(part) : 0;
 		case BLKPG_DEL_PARTITION:
 			part = disk_get_part(disk, partno);
 			if (!part)