HAMMER 34/many: Stabilization pass
* Fix a node reference count leak in hammer_btree_lock_children(). This
was resulting in a panic at umount time.
* Fix a misordered lock in hammer_cursor_up() which could result in
an assertion in the B-Tree iteration code. The cursor-up code was
resolving the parent index prior to acquiring a shared lock on the
parent node. An insertion by another thread could do a split and
change the parent index. The shared lock must be acquired first.
* Add additional debugging output and add additional assertions. HAMMER
now has 205 assert lines in the code.
Reported-by: YONETANI Tomokazu <qhwt+dfly@les.ath.cx> (umount panic)