ide-tape: fix potential fs requests bug

ide-tape had a potential bug for fs requests when preparing the command
packet: it was writing the transfer length as a number of fixed blocks.
However, the block layer implies 512 byte blocks and ide-tape can have
other block sizes so account for that too.

ide-floppy does this calculation properly with the block size factor
(floppy->bs_factor).

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>