fstab-generator: set mode=0755 with root=tmpfs

If mode= is not set in rootflags= add mode=0755 when a tmpfs
is used on the rootfs, otherwise it will be group/world writable
as that's the default mode for tmpfs filesystems.

Follow-up for 725ad3b062

test/test-fstab-generator/test-16-tmpfs.expected/sysroot.mount

@@ -10,4 +10,4 @@ After=imports.target
 What=rootfs
 Where=/sysroot
 Type=tmpfs
-Options=rw
+Options=rw,mode=0755