dumpon: Fix unconfiguring netdump with "off" and "/dev/null".

Netdump has its own configuration tracking such that
ioctl(/dev/null, DIOCSKERNELDUMP) does a dumper_remove() but does not
notify netdump about the removal. Simply sending the same ioctl to
/dev/netdump handles the situation.

Reviewed by: markj, cem
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D31300