Using Zone Append only makes sense for writes to the device, so check
that in btrfs_use_zone_append.  This avoids the possibility of
artificially limited read size on zoned file systems.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>