HAMMER - Fix lost inode issue (primarily with nohistory mounts)

HAMMER - Fix lost inode issue (primarily with nohistory mounts)

* When a HAMMER cursor is unlocked it becomes tracked and unrelated
  B-Tree operations will cause the tracked cursor's nodes and indices
  to be updated.  The cursor structure also has a leaf element pointer
  which was not being properly updated.  This could lead to panics and
  lost inodes.

  Properly adjust the leaf element pointer in tracked cursors.

* The bug primarily occurs with nohistory mounts or nohistory sub-trees
  due to the larger number of physical deletions made to the B-Tree, but
  could also occur (rarely) with normal mounts.

* Add additional assertions to catch any further occurrences (though I
  think all the cases have been covered now).

* Add a new sysctl vfs.hammer.error_panic which can be set to e.g. 9 to
  cause critical errors to panic immediately instead of returning
  through the call stack, making debugging possible.

Reported-by: Numerous people