Another new disk format pushed to -unstable

Hello everyone,

btrfs-unstable now has Yan Zheng's fallocate support, along with disk format changes.

I was hoping to roll all of this into a single format change with the compression code, but there was too much conflict between the two patches.

The fallocate work is pretty neat, it allows preallocation of extents and overwrites them without triggering COW as long as there are no snapshots against the extent.

There should be two sets of disk format changes left before I start trying to avoid them. One to rework the way snapshots and subvolumes are pointed to, and one to put in compat bits so future format changes are all backwards compatible. This will also include moving the superblocks around a bit.

-chris