memory: do_dump_memory_diagnostics(): skip too small small pools
memory: do_dump_memory_diagnostics(): skip too small small pools

We don't use small pools that are smaller than a `free_object`. These
are always empty so skip them.

avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar