Use names suggested by kib@ in review D25969, move call for unmount to not call

Use names suggested by kib@ in review D25969, move call for unmount to not call
with vnode locked, use NOWAIT alloc and only report when we don't overflow.

These changes were accidentally omitted from r364402, except for the not
reporting on overflow. They were lumped in with a debugging commit in my tree
that I omitted w/o realizing this.

Other issues from the review are pending some other changes I need to do first.