btrfs: remove struct btrfs_bio::device field

The device field is only used by the simple end I/O handler, and for
that it can simply be stored in the bi_private field of the bio,
which is currently used for the fs_info that can be retrieved through
bbio->inode as well.

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>