swim: don't call blk_queue_bounce_limit

m68k doesn't support highmem, so don't bother enabling the block layer
bounce buffer code.  Just for safety throw in a depend on !HIGHMEM.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406061725.811389-1-hch@lst.de


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