btrfs: raid56: simplify error handling and code flow in raid56_parity_write

Handle the error return on alloc_rbio failure directly instead of using
a goto and remove the queue_rbio goto label by moving the plugged
check into the if branch.

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>