do not use zram if there's more than 64 GiB free memory (bsc#1197253)

[wfeldt]

Task

Port https://github.com/openSUSE/linuxrc/pull/293 to SLE15-SP4.

Original Problem

• https://bugzilla.suse.com/show_bug.cgi?id=1197253
• https://trello.com/c/8u1xvpxB

On machines with large main memory (above roughly 13 TiB), the kernel breaks if zram is used.

Solution

Limit zram activation to machines with less than 64 GiB free memory. If really needed it can still be activated using the zram=1 boot option.