nfsproto.h: Define the new mode_umask attribute

RFC8275 defines a new attribute as an extension to NFSv4.2
called MODE_UMASK. This patch adds the attribute number
to nfsproto.h.

Future patches will add optional support for the attribute.
This patch does not cause any semantics change.

(cherry picked from commit 10d5b43424da7fadfbdcc41bbffe40d130af1e1d)