jsflash: handle highmem pages

Just kmap the bio single page payload before processing it.

(and yes, now highmem on sparc32 anyway, but kmap_(un)map atomic are nops,
so this gives the right example)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>