Use scatterlists for taskfile based PIO transfers
instead of directly walking rq->[bio,cbio] lists.

If CONFIG_IDE_TASKFILE_IO is defined
this code will be used for fs requests.

ide_pio_sector() is based on ata_pio_sector()
from libata-core.c so kudos to Jeff.
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>