arptable: make slim()'s hunt for packets scale better

Alright, here it is. I tested this on my repro, and it has the desired effect. Success?

Incidentally, master is currently giving me a build error:
../lib/clp.c:43:23: error: conflicting types for ‘uintptr_t’
/usr/include/stdint.h:129:23: note: previous declaration of ‘uintptr_t’ was here