NFSv4: When mounting with a port=0 argument, substitute port=2049

RFC3530 states that the registered port 2049 for the NFS protocol should be
the default configuration in order to allow clients not to use the RPC
binding protocols.
If the mount program sends us a port=0, we therefore substitute port=2049.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>