nfsd: allow passing in array of thread counts via netlink

Now that nfsd_svc can handle an array of thread counts, fix up the
netlink threads interface to construct one from the netlink call
and pass it through so we can start a pooled server the same way we
would start a normal one.

Note that any unspecified values in the array are considered zeroes,
so it's possible to shut down a pooled server by passing in a short
array that has only zeros, or even an empty array.

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