block: stop exporting bio_map_kern

Now that there no module users left of bio_map_kern, stop exporting the
symbol.
Reviewed-by: Javier González <javier@javigon.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hans Holmberg <hans@owltronix.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -1521,7 +1521,6 @@ struct bio *bio_map_kern(struct request_queue *q, void *data, unsigned int len,
 	bio->bi_end_io = bio_map_kern_endio;
 	return bio;
 }
-EXPORT_SYMBOL(bio_map_kern);
 
 static void bio_copy_kern_endio(struct bio *bio)
 {