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: 4e4cbee9 ("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>