silence warning-spam for containers with no memlimit

silence warning-spam for containers with no memlimit

in cgroupv2 environments lxcfs walks up the cgroup tree
until it finds a limit, if it finds none it'll try to
convert an empty string to a number and spam a warning every
time a file requiring this info is accessed

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>