NFSv4: Use the correct net namespace in nfs4_update_server

We need to use the same net namespace that was used to resolve
the hostname and sockaddr arguments.

Fixes: 32e62b7c (NFS: Add nfs4_update_server)
Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>