nfs: make the rpc_stat per net namespace

[ Upstream commit 1548036e ]

Now that we're exposing the rpc stats on a per-network namespace basis,
move this struct into struct nfs_net and use that to make sure only the
per-network namespace stats are exposed.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Stable-dep-of: 24457f1b ("nfs: Handle error of rpc_proc_register() in nfs_net_init().")
Signed-off-by: Sasha Levin <sashal@kernel.org>