Fix shm.exists() check in ingress drop monitor
Fix shm.exists() check in ingress drop monitor

If we are recording ingress drops to a counter, use the right check for
the counter's existence.  Addresses
https://github.com/snabbco/snabb/pull/1047#pullrequestreview-5090751.

snabbco/snabb,mixflowtech/logsensor,alexandergall/snabbswitch,heryii/snabb,dpino/snabb,alexandergall/snabbswitch,Igalia/snabb,heryii/snabb,alexandergall/snabbswitch,snabbco/snabb,eugeneia/snabb,Igalia/snabb,snabbco/snabb,eugeneia/snabb,eugeneia/snabb,dpino/snabb,Igalia/snabb,dpino/snabb,eugeneia/snabbswitch,Igalia/snabb,heryii/snabb,eugeneia/snabbswitch,mixflowtech/logsensor,alexandergall/snabbswitch,Igalia/snabb,snabbco/snabb,kbara/snabb,alexandergall/snabbswitch,mixflowtech/logsensor,Igalia/snabb,eugeneia/snabb,Igalia/snabbswitch,eugeneia/snabb,dpino/snabb,snabbco/snabb,heryii/snabb,Igalia/snabb,snabbco/snabb,dpino/snabb,Igalia/snabbswitch,heryii/snabb,Igalia/snabbswitch,dpino/snabb,kbara/snabb,Igalia/snabbswitch,snabbco/snabb,kbara/snabb,SnabbCo/snabbswitch,dpino/snabbswitch,heryii/snabb,mixflowtech/logsensor,Igalia/snabbswitch,kbara/snabb,eugeneia/snabb,alexandergall/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,kbara/snabb,eugeneia/snabb,eugeneia/snabbswitch,Igalia/snabb,SnabbCo/snabbswitch,eugeneia/snabbswitch,SnabbCo/snabbswitch,mixflowtech/logsensor,alexandergall/snabbswitch,SnabbCo/snabbswitch,dpino/snabb,eugeneia/snabb,dpino/snabbswitch,dpino/snabbswitch,kbara/snabb,dpino/snabbswitch