hammer2 - cleanup, kldload, mount, iget/ifree and other stuff.

hammer2 - cleanup, kldload, mount, iget/ifree and other stuff.

* Make kldload and kldunload run cleanly

* Make mount/umount run cleanly.  Properly deallocate the root inode and
  close the underlying device.

* Write iget/ifree and properly deal with deadlocks between reclaim and
  iget.

* Remove the hm_ prefix for the hammer2_mount structure.

* Use 'hammer2_xxx_t' instead of 'struct hammer2_xxx' in many cases.

* Other general cleanups.