DEBUG: fd: catch access attempts to closed FDs

Certain rare bugs may cause an fd_want_recv() or any of the other
operations being done on a closed FD. This triggers a BUG_ON() on the
next call trying to insert the FD but it's too late to figure when
this happened. Let's just add some BUG_ON_HOT() to detect an attempt
to modify the state of a closed FD so that the culprit is detected.

It will only be enabled with DEBUG_STRICT=2, since by design this may
never happen so it's not needed to enable it in default builds. It was
verified not to trigger on various tests.