dm bufio: fix error code in dm_bufio_write_dirty_buffers()

We should be returning normal negative error codes here.  The "a"
variables comes from &c->async_write_error which is a blk_status_t
converted to a regular error code.

In the current code, the blk_status_t gets propogated back to
pool_create() and eventually results in an Oops.

Fixes: 4e4cbee93d56 ("block: switch bios to blk_status_t")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-bufio.c

@@ -1258,8 +1258,7 @@ EXPORT_SYMBOL_GPL(dm_bufio_write_dirty_buffers_async);
  */
 int dm_bufio_write_dirty_buffers(struct dm_bufio_client *c)
 {
-	blk_status_t a;
-	int f;
+	int a, f;
 	unsigned long buffers_processed = 0;
 	struct dm_buffer *b, *tmp;