i dont think you need that. devip calls a protocol specific
garbage collector procedure that will close these hanging
connections for you if it runs out of connections.

/sys/src/9/ip/tcp.c:/^tcpgc

well, the test is a little bit more complicated as it takes
a minimum inactivity time into account.

made a patch that changes the panic into a print:

/n/sources/patch/devip-allconvused-panic

--
cinap