sys/vfs/hammer: Erase layer2 entries on removing volume

sys/vfs/hammer: Erase layer2 entries on removing volume

Since hammer volume-del puts layer1 entries back to UNAVAIL,
it should zero clear layer2 entries within those layer1s.
We basically want to get rid of all meta data that belonged
to this volume, not just volume header and layer1 entries.

(Layer2 entries in this volume become unreachable data after
layer1 entries lose an offset to layer2s thus should be zero
cleared.)

This commit makes this function look redundant with two full
loops of layer1/2, but this is what's necessary and makes it
more readable plus easier to conform to the original design
than having for-loop function with callback handlers.

Also see d8d6680 and de75bb2.