vfs: allow v_usecount to transition 0->1 without the interlock
There is nothing to do but to bump the count even during said transition.
There are 2 places which can do it:
- vget only does this after locking the vnode, meaning there is no change in contract versus inactive or reclamantion
- vref only ever did it with the interlock held which did not protect against either (that is, it would always succeed)
VCHR vnodes retain special casing due to the need to maintain dev use count.
Reviewed by: jeff, kib
Tested by: pho (previous version)
Differential Revision: https://reviews.freebsd.org/D23185