cgv1: do not disable cpuset manager if reserved interface already exists #16467
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This PR fixes a bug where restarting a Nomad Client on a machine using cgroups
v1 (e.g. Ubuntu 20.04 or earlier) would cause the cpuset cgroups manager to disable itself.
This is being caused by incorrectly interpreting a "file exists" error as
problematic when ensuring the reserved cpuset exists. If we get a "file exists"
error, that just means there is nothing to do, and the cpuset manager should continue
initializing normally.
Note that a machine reboot would fix the issue - the interfaces are ephemeral.
Fixes #16291