NFS: Use the NFS_DEFAULT_VERSION for v2 and v3 mounts

Older versions of nfs utils don't always pass a "vers=" mount option for
NFS.  This chould lead to attempts at using NFS v0 due to a zeroed out
nfs_parsed_mount_data struct.  I solve this by setting the default NFS
version to NFS_DEFAULT_VERSION in the v2 and v3 cases (v4 has already been
taken care of by a similar patch).

Reported-by: Joerg Roedel <joro@&bytes.org>
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>