Use "tmpfs" instead of "/dev/shm" as tmpfs device name

A tmpfs does not have a device, so usually, "tmpfs" is used as the device name.

When "/dev/shm" is used instead, this confuses systemd, which repeatedly prints this error in system logs:

Failed to open /dev/shm device, ignoring: Inappropriate ioctl for device

This is because systemd looks through /proc/self/mounts, sees /dev/shm and tries to open it as a device, which fails because /dev/shm is not a device. You can reproduce this problem by booting a debian live system with toram boot option.

By changing the tmpfs device to "tmpfs", we can remove this noise from system logs.