ide: add ide_atapi_{discard_data,write_zeros} inline helpers

Add ide_atapi_{discard_data,write_zeros} inline helpers to <linux/ide.h>
and use them instead of home-brewn helpers in ide-{floppy,tape,scsi}.

There should be no functional changes caused by this patch.

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>