It only took 5 years... (finished the block 'extra data' work)

[Barret Rhoden]

Hi -

Remember that struct block 'extra data' stuff that Drew and I started
nearly 5 years ago? Done. The last of the PANIC_EXTRA()s are gone.

For most of those years, there were just a couple of corner cases where
we'd panic, and we just never ran into them - until I started mounting
9p connections in a certain way. You also had the ability to build
the kernel without it - that was because early versions of the code
(2015) broke Kevin's Go work.

Anyway, it took a couple days to sort out fun functions like padblock,
pullupblock, concatblock, etc, and there are probably bugs with it that
syzkaller will find. But I got rid of a TODO. =)

Barret