NFS: Remove v3 not compiled check from validate_mount_data()

This check is already performed by the module loading code - if the
module can't be found then -EPROTONOSUPPORT will be returned.  Let's
handle v3 this way, too.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>