btrfs: never return true for reads in btrfs_use_zone_append

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>

fs/btrfs/zoned.c

@@ -1641,6 +1641,9 @@ bool btrfs_use_zone_append(struct btrfs_bio *bbio)
 	if (!is_data_inode(&inode->vfs_inode))
 		return false;
 
+	if (btrfs_op(&bbio->bio) != BTRFS_MAP_WRITE)
+		return false;
+
 	/*
 	 * Using REQ_OP_ZONE_APPNED for relocation can break assumptions on the
 	 * extent layout the relocation code has.