SUNRPC: register RPC stats /proc entries in passed network namespace context

This patch makes it possible to create NFS program entry ("/proc/net/rpc/nfs")
in passed network namespace context instead of hard-coded "init_net".

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>