NFS: Fix port initialisation in nfs_remount()

The recent changeset 53a0b9c4

 (NFS: Replace
nfs_parse_ip_address() with rpc_pton()) broke nfs_remount, since the call
to rpc_pton() will zero out the port number in data->nfs_server.address.

This is actually due to a bug in nfs_remount: it should be looking at the
port number in nfs_server.port instead...

This fixes bug
   http://bugzilla.kernel.org/show_bug.cgi?id=14276

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