blkdev_issue_zeroout try discard/writesame first, if they fail, zeroout
fallback to regular write. The problem is discard/writesame doesn't
return error for -EOPNOTSUPP, then zeroout can't do fallback and leave
disk data not changed. zeroout should have guaranteed zero-fill
BTW, I saw several callers of blkdev_issue_discard can handle
-EOPNOTSUPP, not sure why blkdev_issue_discard not returns -EOPNOTSUPP.
The same story for blkdev_issue_write_same.