nfsd: make nfsd_svc take an array of thread counts

Now that the refcounting is fixed, rework nfsd_svc to use the same
thread setup as the pool_threads interface. Have it take an array of
thread counts instead of just a single value, and pass that from the
netlink threads set interface.  Since the new netlink interface doesn't
have the same restriction as pool_threads, move the guard against
shutting down all threads to write_pool_threads.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>