There is a case when B_VMIO is clear where a buffer can be placed on the

There is a case when B_VMIO is clear where a buffer can be placed on the
EMPTY or EMPTYKVA queues without being disassociated from its vnode.
This can lead to a duplicate logical block panic in the red-black tree code.
Rework brelse() to ensure that buffers are properly cleaned up before being
placed on said queues, and add assertions to validate other cases.

Reported-by: Tomaz Borstnar