SUNRPC: register PipeFS file system after pernet sybsystem

PipeFS superblock creation routine relays on SUNRPC pernet data presense, which
is created on register_pernet_subsys() call in SUNRPC module init function.
Registering of PipeFS filesystem prior to registering of per-net subsystem
leads to races (mount of PipeFS can dereference uninitialized data).

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