Re: [BUG v4.10-rc1] fzero returns EIO on DAX mount

The problem is that brd return -EIO due to alignment, and the old
code waited for the bio and then ignored the error.  The new code
assumes if the submission is fine this is the method to pick and to
move on.  We could revert some of this for now, but I think the right
thing to do is to stop abusing REQ_OP_DISCARD for zeroing now.

I had planned to do this for 4.10 but it splitted, let's see if I
can get the series small enough for this time in the merge window.