btrfs: fix a typo in btrfs_use_zone_append

REQ_OP_ZONE_APPNED -> REQ_OP_ZONE_APPEND.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>