swim3: support highmem

swim3 only uses the virtual address of a bio to stash it into the data
transfer using virt_to_bus.  But the ppc32 virt_to_bus just uses the
physical address with an offset.  Replace virt_to_bus with a local hack
that performs the equivalent transformation and stop asking for block
layer bounce buffering.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406061839.811588-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>