bcachefs - snapshots

[Kent Overstreet]

Snapshots have been merged! 9 months of work and 3k lines of new code, finally
released. Some highlights:

 - btrfs style subvolumes & snapshots interface
 - snapshots are writeable
 - highly scalable: number of snapshots is limited only by your disk space
 - highly space efficient: no internal fragmentation issues

Design doc here: https://bcachefs.org/Snapshots/

The core functionality is complete - snapshot creation and deletion works, fsck
changes are done (most of the complexity was in making fsck work without
O(number of snapshots) performance - tricky). Everything else is a todo item:

 - still need to export different st_dev for files in different subvolumes
   (we'll never allocate a new inode with an inode number that collides with an
   inode inother subvolume - but snapshots will naturally result in colliding
   inode numbers)

 - need to hide dirents that point to snapshots when inside snapshots...

 - snapshot creation is not atomic w.r.t. page cache, we do sync_fs() but don't
   block buffered writes

 - other niggling page cache stuff - need to walk page cache and mark blocks as
   no longer reserved on snapshot creation

 - we no longer have quota support, since old style quotas interact badly with
   snapshots

 - we need per subvolume disk space accounting before i can implement btrfs
   style subvolume quotas

 - all the things I neglected to think of yet, and all the bugs I haven't found
   yet

Go wild, please try and break it.