The vinactive() call in vgonel() may start writes for the dirty pages,
creating delayed write buffers belonging to the reclaimed vnode. Put
the buffer cleanup code after inactivation.

Add asserts that ensure that buffer queues are empty and add BO_DEAD
flag for bufobj to check that no buffers are added after the cleanup.
BO_DEAD is only used by INVARIANTS-enabled kernels.

Reported and tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week