HAMMER 61A/Many: Stabilization

HAMMER 61A/Many: Stabilization

* Fix a buffer exhaustion issue.  When creating large numbers of empty files
  a single inode sync of the directory can exhaust the buffer cache because
  not enough other things get dirty enough to force a flush.

  Put a check in the record syncing code and do a partial finalization if
  necessary to avoid deadlocking the buffer cache.

* Fix a panic caused by a missing call to hammer_cursor_deleted_element().

Reported-by: Michael Neumann <mneumann@ntecs.de>,
	     Gergo Szakal <bastyaelvtars@gmail.com>