Crashes have been observed for NFSv4.1 mounts when the system
is being shut down which were caused by the nfscbd_pool being
destroyed before the backchannel is disabled. This patch is
believed to fix the problem, by simply avoiding ever destroying
the nfscbd_pool. Since the NFS client module cannot be unloaded,
this should not cause a memory leak.

MFC after:	2 weeks