nfsd: Add VNET_SYSUNINIT() macros for vnet cleanup

Commit ed03776ca7f4 enabled the vnet front end macros.
As such, for kernels built with the VIMAGE option will malloc
data and initialize locks on a per-vnet basis, typically
via a VNET_SYSINIT().

This patch adds VNET_SYSUNINIT() macros to do the frees
of the per-vnet malloc'd data and destroys of per-vnet
locks. It also removes the mtx_lock/mtx_unlock calls
from nfsrvd_cleancache(), since they are not needed.

Discussed with: bz, jamie
MFC after: 3 months