ide: fix ->io_32bit race in ide_taskfile_ioctl()

In ide_taskfile_ioctl(), there was a race condition involving
drive->io_32bit.  It was cleared and restored during ioctl
requests but there was no synchronization with other requests.
So, other requests could execute with the altered ->io_32bit
setting or updated drive->io_32bit could be overwritten by
ide_taskfile_ioctl().

This patch adds IDE_TFLAG_IO_16BIT flag to indicate to
ide_pio_datablock() that 16-bit I/O is needed regardless of
drive->io_32bit settting.

Bart:
- ported it over recent IDE changes

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>