Ensure that we fix up the missing fields in the nfs_mount_data with
 sane defaults for older versions of mount, and return errors in the
 cases where we cannot.

 Convert a bunch of annoying warnings into dprintks()

 Return -EPROTONOSUPPORT rather than EIO if mount() tries to set NFSv3
 without it actually being compiled in.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>