Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
I'm for solution 3:
- keep blk_bio_{discard,write_same}_split, but ensure we never built
a > 4GB bio in blkdev_issue_{discard,write_same}.
Note that this isn't special casing, we can't build > 4GB bios for
data either, it's just implemented as a side effect right now instead
of checked explicitly.