btrfs: don't use bio->bi_private to pass the inode to submit_one_bio

submit_one_bio is only used for page cache I/O, so the inode can be
trivially derived from the first page in the bio.

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