ng_ksocket: fix upcall clearing on node shutdown

Note: imho, the proper solution would be to guarantee that upcalls
won't ever be called after soclose(), but this isn't the case, yet.
This change at least makes the node work the way it always worked.

Reviewed by: afedorov
Differential Revision: https://reviews.freebsd.org/D42636
PR: 272319
PR: 275106
Fixes: 779f106aa169256b7010a1d8f963ff656b881e92