client: do not disable memory swappiness if kernel does not support it

[shoenig]

This PR adds a workaround for very old Linux kernels which do not support
the memory swappiness interface file. Normally we write a "0" to the file
to explicitly disable swap. In the case the kernel does not support it,
give libcontainer a nil value so it does not write anything.

Fixes #17448

[shoenig]

Spot check we on 20.04 (cgroups v1) that we didn't break anything

ubuntu@ip-172-31-4-13:~$ sudo ./nomad/bin/nomad version
Nomad v1.6.0-dev
BuildDate 2023-06-21T13:20:42Z
Revision 76e419aec82ae7a2e8c909adbff3c299bab3252a+CHANGES

checking our agent detects cgroups v1

ubuntu@ip-172-31-4-13:~$ ./nomad/bin/nomad node status -self -verbose | grep cgroup
unique.cgroup.mountpoint                 = /sys/fs/cgroup/systemd
unique.cgroup.version                    = v1

swappiness is still set on cgroups v1 on newer kernels

ubuntu@ip-172-31-4-13:~/nomad/client/lib/cgutil$ sudo /usr/local/go/bin/go test -v -run TestUtil_MaybeDisableMemorySwappiness
=== RUN   TestUtil_MaybeDisableMemorySwappiness
=== PAUSE TestUtil_MaybeDisableMemorySwappiness
=== CONT  TestUtil_MaybeDisableMemorySwappiness
--- PASS: TestUtil_MaybeDisableMemorySwappiness (0.00s)
PASS
ok      github.com/hashicorp/nomad/client/lib/cgutil    0.019s

running exec tests

ubuntu@ip-172-31-4-13:~/nomad/drivers/exec$ sudo /usr/local/go/bin/go test 1>/dev/null 2>&1
ubuntu@ip-172-31-4-13:~/nomad/drivers/exec$ echo $?
0

[jrasell]

LGTM.