btrfs: simplify code flow in btrfs_submit_dio_bio

There is no exit block and cleanup and the function is reasonably short
so we can use inline return and not the goto. This makes the function
more straight forward.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>