can you take at my (fully working, but not fully cleaned up) version
of the iomap based DAX code here:
By using iomap we don't even have the size hole problem and totally
get out of the reverse-engineer what buffer_heads are trying to tell
us business. It also gets rid of the other warts of the DAX path
due to pretending to be like direct I/O, so this might be a better
way forward also for ext2/4.