vfs: release usecount using fetchadd

1. If we release the last usecount we take ownership of the hold count, which

means the vnode will remain allocated until we vdrop it.

2. If someone else vrefs they will find no usecount and will proceed to add

their own hold count.

3. No code has a problem with v_usecount transitioning to 0 without the

interlock

These facts combined mean we can fetchadd instead of having a cmpset loop.

Reviewed by: kib (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21528