nfs: expose /proc/net/sunrpc/nfs in net namespaces

We're using nfs mounts inside of containers in production and noticed
that the nfs stats are not exposed in /proc.  This is a problem for us
as we use these stats for monitoring, and have to do this awkward bind
mount from the main host into the container in order to get to these
states.

Add the rpc_proc_register call to the pernet operations entry and exit
points so these stats can be exposed inside of network namespaces.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>