Fix incorrect double-termination of vm_object.  When a vm_object is terminated and flushes pending dirty pages it is possible for the object to be ref'd (0->1) and then deref'd (1->0) during termination. We do not terminate the object a second time.
Fix incorrect double-termination of vm_object.  When a vm_object is
terminated and flushes pending dirty pages it is possible for the
object to be ref'd (0->1) and then deref'd (1->0) during termination.
We do not terminate the object a second time.

Document vop_stdgetvobject() to explicitly allow it to be called without
the vnode interlock held (for upcoming sync_msync() and ffs_sync()
performance optimizations)

MFC after:	3 days

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