hammer2 - Implement error processing and free reserve enforcement

hammer2 - Implement error processing and free reserve enforcement

* newfs_hammer2 calculates the correct amount of reserved space.  We
  have to reserve 4MB per 1GB, not 4MB per 2GB, due to a snafu.  This
  is still only 0.4% of the storage.

* Flesh out HAMMER2_ERROR_* codes and make most hammer2 functions return
  a proper error code.

* Add error handling to nearly all code that can dirty a chain, in
  particular to handle ENOSPC issues.  Any dirty buffers that cannot be
  flushed will incur a write error (which in DragonFly typically causes
  the buffer to be retries later).  Any dirty chain that cannot be
  flushed will remain in the topology and can be completed in a later
  flush if space has been freed up.

  We try to avoid allowing the filesystem to get into this situation in
  the first place, but if it does, it should be possible to flush these
  asynchronous modifying chains and buffers once space is freed up via
  bulkfree.

* Relax class match requirements in the freemap allocator when the freemap
  gets close to full.  This will allow e.g. inodes to be allocated out of
  DATA bitmaps and vise versa, and so forth.  This fixes edge conditions
  where there is enough free space available but it has all been earmarked
  for the wrong data class.

* Try to fix a bug in live_count tracking when destroying an indirect
  block chain or inode chain that has not yet been blockmapped due to
  a drop.  This situation only occurs when chains cannot be flushed due
  to I/O errors or disk full conditions, and are then later destroyed
  (e.g. such as when the governing file is removed).

  This should fix a live_count assertion that can occur under these
  circumstances.  See hammer2_chain_lastdrop().

* Enforce the free reserve requirement for all modifying VOP calls.
  Root users can nominally fill the file system to 97.5%, non-root
  users to 95%.  At 90%, write()s will enforce bawrite() verses bdwrite()
  to try to avoid buffer cache flushes from actually running the
  filesystem out of space.

  This is needed because we do not actually know how much disk space is
  going to be needed at write() time.  Deduplication and compression
  occurs later, at buffer-flush time.

* Do NOT flush the volume header when a vfs sync is unable to completely
  flush a device due to errors.  This ensures that the underlying media
  does not become corrupt.

* Fix an issue where bref.check.freemap.bigmask was not being properly
  reset to -1 when bulkfree is able to free an element.  This bug
  prevented the allocator from recognizing that free space was available
  in that bitmap.

* Modify bulkfree operation to use the live topology when flushing and
  snapshot operations fail due to errors, allowing bulkfree to run.

* Nominal bulkfree operations now runs on the snapshot without a
  transaction (more testing is needed).  This theoretically should allow
  bulkfree to run concurrent with just about any operation including
  flushes.

* Add a freespace tracking heuristic to reduce the overhead that modifying
  VOP calls incur in checking the free reserve requirement.

* hammer2 show dumps additional info for freemap nodes.