npf: Don't stop early after sleeping and before processing instances.

We already check winfo->exit below, after processing instances and
before sleeping again.

Candidate fix for:

panic: kernel diagnostic assertion "LIST_EMPTY(&winfo->instances)" failed: file "/home/riastradh/netbsd/current/src/sys/rump/net/lib/libnpf/../../../..//net/npf/npf_worker.c", line 300 NPF instances must be discharged before the npfk_sysfini() call

(riastradh)