btrfs: pass a btrfs_bio to btrfs_repair_one_sector

Pass the btrfs_bio instead of the plain bio to btrfs_repair_one_sector,
and remove the start and failed_mirror arguments in favor of deriving
them from the btrfs_bio.  For this to work ensure that the file_offset
field is also initialized for buffered I/O.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>