Handle most object reference count updates without object locks.

[jeff]

Summary

This patch moves to only requiring the object lock on the last ref (or second to last for anonymous objects) as well as the first ref of a vnode object.

The object lock wouldn't be required for the first vnode ref except to serialize with vnode_pager_dealloc() which could race and drop a vref that is not yet acquired. I'm not sure if this is even possible anymore and if it is not vm_object_reference() could be simplified.

[jeff]

sys/vm/vnode_pager.c

176 ↗

(On Diff #64598)

s/thee/the/

[jeff]

Fix a race between unlocked reference and collapse.

This now passes various stress tests and I believe it is suitable to go in. The reduction in locking of the vnode object is substantial.