insufficient at this moment and we can bind(2) two sockets listen on same
port number.
for real fix, we need to check inpcb table with in6pcb. we can't
find inpcb chain from particular in6pcb chain (like finding tcbtable from tcb6)
luckily RFC2553 does not talk about bind(2) behavior for IPv4 mapped.
IPv4 mapped brings in too much complexities...
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