floppy: always use the track buffer

floppy: always use the track buffer

Always use the track buffer that is already used for addresses outside
the 16MB address capability of the floppy controller.  This allows to
remove a lot of code that relies on kernel virtual addresses.  With
this gone there is just a single place left that looks at the bio,
which can be converted to memcpy_{from,to}_page, thus removing the need
for the extra block-layer bounce buffering for highmem pages.

Signed-off-by: Christoph Hellwig <hch@lst.de>