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).