SUNRPC: hold current network namespace while pipefs superblock is active

We want to be sure that network namespace is still alive while we have pipefs
mounted.
This will be required later, when RPC pipefs will be mounting only from
user-space context.

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