Cleanup etc. from reading the f2fs LKML discussion

[stevenjswanson]

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/