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.

MFC after:	2 weeks

sys/fs/nfs/nfsproto.h

@@ -1004,7 +1004,7 @@ struct nfsv3_sattr {
 #define	NFSATTRBIT_SPACEFREED		78
 #define	NFSATTRBIT_CHANGEATTRTYPE	79
 #define	NFSATTRBIT_SECLABEL		80
-/* Not sure what attribute bit #81 is? */
+#define	NFSATTRBIT_MODEUMASK		81
 #define	NFSATTRBIT_XATTRSUPPORT		82
 
 #define	NFSATTRBM_SUPPORTEDATTRS	0x00000001
@@ -1088,7 +1088,7 @@ struct nfsv3_sattr {
 #define	NFSATTRBM_SPACEFREED		0x00004000
 #define	NFSATTRBM_CHANGEATTRTYPE	0x00008000
 #define	NFSATTRBM_SECLABEL		0x00010000
-/* Not sure what attribute bit#81/0x00020000 is? */
+#define	NFSATTRBM_MODEUMASK		0x00020000
 #define	NFSATTRBM_XATTRSUPPORT		0x00040000
 
 #define	NFSATTRBIT_MAX			83