Error handling patchset v6

Here's my current error handling patchset, against Chris's current for-linus branch.

As before, it's almost all in preparation for actual error handling. After a chat with Chris last week about some of the bits I thought might be contentious, I went ahead with the next step and converted struct extent_state allocations to use a mempool in front of the slab cache it was using already. This means that non-atomic callers can't catch an -ENOMEM and that means that non-atomic version of {set,clear,convert}_extent_bit can return void. This changes the previous patch set quite a bit since the {set,clear,convert}_extent_bit error push-up patches are now entirely obsolete.

The run down:
- extent_state allocations -> mempool
- delayed_ref allocations -> mempool
- a ton of int -> void changes
- add btrfs_panic
- a few cleanups where I ran into them

-Jeff