In vgonel(), postpone setting BO_DEAD until VOP_RECLAIM() is called,
if vnode is VMIO. For VMIO vnodes, set BO_DEAD in vm_object_terminate().

The vnode_destroy_object(), when calling into vm_object_terminate(),
must be able to flush buffers. BO_DEAD purpose is to quickly destroy
buffers on write when the underlying vnode is not operable any more
(one example is the devfs node after geom is gone). Setting BO_DEAD
for reclaiming vnode before object is terminated is premature, and
results in unability to flush buffers with live SU dependencies from
vinvalbuf() in vm_object_terminate().

Reported by: David Cross <dcrosstech@gmail.com>
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks