block: add a error_count field to struct request

This is for the legacy floppy and ataflop drivers that currently abuse
->errors for this purpose.  It's stashed away in a union to not grow
the struct size, the other fields are either used by modern drivers
for different purposes or the I/O scheduler before queing the I/O
to drivers.

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

include/linux/blkdev.h

@@ -175,6 +175,7 @@ struct request {
 		struct rb_node rb_node;	/* sort/lookup */
 		struct bio_vec special_vec;
 		void *completion_data;
+		int error_count; /* for legacy drivers, don't use */
 	};
 
 	/*