hammer2 - freemap and data check code
* Change the freemap to use 8 sets = ~32 reserved blocks instead of 14 sets.
Use a straight iterator. (TODO - must invalidate newer volumes if an older
volume backup is mounted RW because freemap will become inconsistent).
* Update the freemap documentation. Requires more work.
* MODIFY_OPTDATA can only skip to the end if chain->data is NULL in
the BREF_TYPE_DATA case. chain->data might not be NULL if the
data chain represents a compressed device buffer.
* Add check code generation and testing. Currently just kprintf()s a
warning if the check code does not match. Implement iscsi crc-32,
sha-192 (this is sha-256 with the last 64 bits XOR'd into the first),
and implement check generation and testing for all data, meta-data,
and the freemap. For Everything. 64-bit CRCs are not yet implemented.
* Fix a few cases where the DIO or the chain was not being properly
dirtied prior to making modifications to the media contents via
chain->data (this showed up because CRC tests were failing).
* Cleanup:
- CHAIN_FORCECOW is no longer used, remove it.
- Move FLUSH_DEPTH_LIMIT from hammer2.h to hammer2_flush.c.
- Remove debug structures from the blockref check union.
- Remove some #if 0'd code that is no longer applicable.