r334853 added a "socket destructor" callback. However, as implemented, it
was really a "socket close" callback.

Update the socket destructor functionality to run when a socket is
destroyed (rather than when it is closed). The original submitter has
confirmed that this change satisfies the intended use case.

Suggested by: rwatson
Submitted by: Michio Honda <micchie at sfc.wide.ad.jp>
Tested by: Michio Honda <micchie at sfc.wide.ad.jp>
Approved by: re (kib)
Differential Revision: https://reviews.freebsd.org/D17590