hammer2 - Work on concurrent bulkfree stability
* The dedup bits in the DIO structure must be set atomically with
the setting of the bitmap bits in the freemap to avoid racing
bulkfree.
* dedup bits are normally deleted on the 11->10 transition in bulkfree,
and asserted to be deleted on the 10->00 transition. Also delete
dedup bits when a modified chain with no parent is destroyed, and
for a chain's prior data reference when it is being replaced by a
new data reference.
* Clean up the DIO dedup management code.
* Clean up the DIO allocation code.
* Cap the size of the DIO cache to 100000 elements for now, if nbuf * 2
is greater.