btrfs: pass a btrfs_bio to btrfs_use_append

struct btrfs_bio has all the information needed for btrfs_use_append, so
pass that instead of a btrfs_inode and file_offset.

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