NFS: Ensure struct nfs_client holds a reference to the net namespace

Otherwise we have no guarantee that the net namespace won't just
disappear from underneath us once the task that created it
is destroyed.

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