tcptop: fix display of received bytes, reduce syscalls (fixes #1871)

the keys variable was a reference to ipv4_recv_bytes, and after merging
the keys of ipv4_send_bytes the original ipv4_recv_bytes BPF table also
contained all keys, therefore the `if k in ipv4_recv_bytes` check a few
lines below always evaluates to true

with this commit all BPF tables are copied to userspace as dicts once
(to reduce the number of syscalls) and the keys are merged in a new dict