Making the superblock look like "just another entry" allows us to treat
the superblock like "just another entry" and reuse a decent amount of
logic that would otherwise only be used a format and mount time. In this
case we can use append to write out the superblock like it was creating
a new entry on the filesystem.
This is only an issue in the weird case that are worn down block is
left in the odd state of not being able to change the data that resides
on the block. That being said, this does pop up often when simulating
wear on block devices.
Currently, directory commits checked if the write succeeded by crcing the
block to avoid the additional RAM cost for another buffer. However,
before this commit, directory commits just checked if the block crc was
valid, rather than comparing to the expected crc. This would usually
work, unless the block was stuck in a state with valid crc.
The fix is to simply compare with the expected crc to find errors.
This provides a limited form of wear leveling. While wear is
not actually balanced across blocks, the filesystem can recover
from corrupted blocks and extend the lifetime of a device nearly
as much as dynamic wear leveling.
For use-cases where wear is important, it would be better to use
a full form of dynamic wear-leveling at the block level. (or
consider a logging filesystem).
Corrupted block handling was simply added on top of the existing
logic in place for the filesystem, so it's a bit more noodly than
it may have to be, but it gets the work done.
