NFS: Create a single function for text mount data

The v2/3 and v4 cases were very similar, with just a few parameters
changed.  This makes it easy to share code.

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