stats: map MaxUsage to the correct value

and make sure it is not set for cgroup v2

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

test/apiv2/20-containers.at

@@ -98,6 +98,12 @@ else
     fi
 fi
 
+# max_usage is not set for cgroupv2
+if have_cgroupsv2; then
+    t GET libpod/containers/stats?containers='[$cid]' 200 \
+    .memory_stats.max_usage=null
+fi
+
 t DELETE libpod/containers/$cid 200 .[0].Id=$cid
 
 # Issue #14676: make sure the stats show the memory limit specified for the container