rbd: end I/O the entire obj_request on error

When we end I/O struct request with error, we need to pass
obj_request->length as @nr_bytes so that the entire obj_request worth
of bytes is completed. Otherwise block layer ends up confused and we
trip on

rbd_assert(more ^ (which == img_request->obj_request_count));

in rbd_img_obj_callback() due to more being true no matter what. We
already do it in most cases but we are missing some, in particular
those where we don't even get a chance to submit any obj_requests, due
to an early -ENOMEM for example.

A number of obj_request->xferred assignments seem to be redundant but
I haven't touched any of obj_request->xferred stuff to keep this small
and isolated.

Cc: Alex Elder <[e-mail]>
Cc: [e-mail] # 3.10+
Reported-by: Shawn Edwards <[e-mail]>
Reviewed-by: Sage Weil <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

drivers/block/rbd.c

···
 			result, xferred);
 		if (!img_request->result)
 			img_request->result = result;
+		/*
+		 * Need to end I/O on the entire obj_request worth of
+		 * bytes in case of error.
+		 */
+		xferred = obj_request->length;
 	}
 
 	/* Image object requests don't own their page array */