hammer2 - serialized flush work part 4

hammer2 - serialized flush work part 4

* Hardlink helper functions now take and return hammer2_chain pointers
  instead of messing with hammer2_inode's, which simplifies matters
  greatly.

* Major cleanup of the hardlink handling code.

* Get hardlink creation/deletion mostly working as expected again.
  There is still an issue with hammer_inode aliasing (see TODO),
  and the rename-directory-to-some-parent hardlink issue has not been
  addressed yet.