Net: Increase `NET_BUF_USER_DATA_SIZE` value to 8

Increase `NET_BUF_USER_DATA_SIZE` value to 8 if `BT_CONN` is enabled.
This is necessary because one of the backported commits adds one struct
member into `struct tx_meta`, and that will be stored in the buffer
user data.

On main, this Kconfig option has been deprecated, hence why this change
was not present in the original patch set.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>