Cleanup etc. from reading the f2fs LKML discussion

https://lkml.org/lkml/2012/10/22/664

- Add buffer before superblock begin? (https://lkml.org/lkml/2012/10/26/246)
- Should we have a volume serial number?
- comments on every data structure.
- move userspace visible data structures (e.g., for fsk) to include/linux
- add magic number.  Put it in magic.h
- explain magical constants.
- what belongs in /sys/fs vs debugfs vs. /proc/
