btrfs: track the next file offset in struct btrfs_bio_ctrl

The bio implementation is not something we should really mess around,
and we shouldn't recalculate the pos from the folio over and over.
Instead just track then end of the current bio in logical file offsets
in the btrfs_bio_ctrl, which is much simpler and easier to read.

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