Include MemSpecLimit when calculating defmem

[wiktorn]

To prevent OOMKiller killing random processes on the node it is possible to define MemSpecLimit which reserves some of the memory for the system and limit job memory below what is available on the node.

This example reserves 1024MB of RAM for system on the node:

      - id: debug_nodeset
        source: community/modules/compute/schedmd-slurm-gcp-v6-nodeset
        use: [network]
        settings:
          disk_size_gb: 30
          machine_type: n2d-standard-2
          node_conf:
            MemSpecLimit: 1024

But with such definition, running job fails with:

$ srun -p debug hostname
srun: error: Unable to allocate resources: Requested node configuration is not available

Though running job with:

$ srun -p debug --mem 100 hostname

Succeeds, as it requests less memory.

This change subtracts reserved memory from total memory available on the instance before calculating DefMemPerCPU which results in default memory claim within available memory.

This is most visible on 1 CPU nodes, but with larger nodes, at least one CPU may not be available for scheduling due to this.

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