btrfs: fix uninitialized variable warning in run_one_async_start

With -Wmaybe-uninitialized compiler complains about ret being possibly
uninitialized, which isn't possible as the WQ_ constants are set only
from our code, however we can handle the default case and get rid of the
warning.

The value is set to BLK_STS_IOERR so it does not issue any IO and could
be potentially detected, but this is basically a "cannot happen" error.
To catch any problems during development use the assert.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ set the error in default: ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -717,6 +717,10 @@ static void run_one_async_start(struct btrfs_work *work)
 		ret = btrfs_submit_bio_start_direct_io(async->inode,
 				async->bio, async->dio_file_offset);
 		break;
+	default:
+		/* Can't happen so return something that would prevent the IO. */
+		ret = BLK_STS_IOERR;
+		ASSERT(0);
 	}
 	if (ret)
 		async->status = ret;