NFS: Don't apply NFS_MOUNT_FLAGMASK to text-based mounts

The point of introducing text-based mounts was to allow us to add
functionality without having to worry about legacy binary mount formats.
The mask should be there in order to ensure that binary formats don't start
enabling features that they cannot support. There is no justification for
applying it to the text mount path.

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