btrfs: don't opencode sync_blockdev in btrfs_init_new_device

Instead of opencoding filemap_write_and_wait simply call syncblockdev as
it makes it abundantly clear what's going on and why this is used. No
semantics changes.
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2512,7 +2512,7 @@ int btrfs_init_new_device(struct btrfs_fs_info *fs_info, const char *device_path
 		locked = true;
 	}
 
-	filemap_write_and_wait(bdev->bd_inode->i_mapping);
+	sync_blockdev(bdev);
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {