SUNRPC: set per-net PipeFS superblock before notification

There can be a case, when on MOUNT event RPC client (after it's dentries were
created) is not longer hold by anyone except notification callback.
I.e. on release this client will be destoroyed. And it's dentries have to be
destroyed as well. Which in turn requires per-net PipeFS superblock to be set.

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