How to move or link /tmp folder to existing partition mount?

I have an additional partition mounted at /data:

/dev/mapper/mypartition /data ext4 default 0 1

Now I want to also move my /tmp folder to /data/tmp.

I tried by adding another fstab entry:

/dev/mapper/mypartition/tmp /tmp none default,bind 0 0

But that does not work and prevented my machine from booting. How could this be achieved?

My goal is to copy the tmp folder to the new partition. Because my boot partition is 30GB, and my new partition 200Gb.