block: add a error_count field to struct request

[hch at lst.de]

On Tue, Apr 18, 2017 at 10:57:11PM +0000, Bart Van Assche wrote:
> Both blk-mq and the traditional block layer support a .cmd_size field to
> make the block layer core allocate driver-specific data at the end of struct
> request. Could that mechanism have been used for the error_count field?

It could, and that's what I did for the modern drivers.  It would have
been a bit of a pain for these old floppy drivers, though.