nfsd: make svc_stat per-network namespace instead of global

The final bit of stats that is global is the rpc svc_stat.  Move this
into the nfsd_net struct and use that everywhere instead of the global
struct.  Remove the unused global struct.

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