[CIFS] Allow disabling CIFS Unix Extensions as mount option

Previously the only way to do this was to umount all mounts to that server,
turn off a proc setting (/proc/fs/cifs/LinuxExtensionsEnabled).

Fixes Samba bugzilla bug number: 4582 (and also 2008)

Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsglob.h

@@ -281,7 +281,9 @@ struct cifsTconInfo {
 	FILE_SYSTEM_UNIX_INFO fsUnixInfo;
 	unsigned retry:1;
 	unsigned nocase:1;
-	/* BB add field for back pointer to sb struct? */
+	unsigned unix_ext:1; /* if off disable Linux extensions to CIFS protocol
+				for this mount even if server would support */
+	/* BB add field for back pointer to sb struct(s)? */
 };
 
 /*