floppy: cleanup: expand the reply_buffer macros

Several macros were used to access reply_buffer[] at discrete positions
without making it obvious they were relying on this. These ones have
been replaced by their offset in the reply buffer to make these accesses
more obvious.

Link: https://lore.kernel.org/r/20200224212352.8640-11-w@1wt.eu


Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>