Make sure that we refresh the PID on read(2) and write(2) operations.
This fixes the process portion of the bpf(4) stats if the peer forks
into the background after it's opened the descriptor. This bug
results in the following behavior for netstat -B:

netstat -B

Pid  Netif  Flags      Recv      Drop     Match Sblen Hblen Command

netstat: kern.proc.pid failed: No such process
78023 em0 p--s-- 2237404 43119 2237404 13986 0 ??????

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