hammer2 - Increase size of blockref, refactor freemap.

hammer2 - Increase size of blockref, refactor freemap.

I finally buckled under and increase the size of the blockref from 64 bytes
to 128 bytes.  The negative consequences are relatively minor, and the
positives are many.

Negative consequences

    * 512 blocks per 64KB indirect block instead of 1024.
    * Inode has only 4 built-in blockrefs instead of 8.
    * Indirect block overhead is doubled.
    * Freemap overhead is doubled due to SEGSIZE being 1GB instead of 2GB.
    * Small files will have to push an indirect block sooner (at 256KB).

Positive consequences

    * Freemap overhead is still only 0.40% of the storage.
    * Freemap now uses a 32KB indirect block instead of a 64KB indirect block.

    * Data and inode statistics can be tracked in the blockref.
    * Check code goes from max-192-bits to max-512-bits.
    * Additional fields now available for future filesystem features.