Use atomics to modify numvnodes variable.

This allows to mostly avoid lock usage in getnewvnode_[drop_]reserve(),
that reduces number of global vnode_free_list_mtx mutex acquisitions
from 4 to 2 per NFS request on ZFS, improving SMP scalability.

Reviewed by:	kib
MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.