This change fixes lazy socket descriptor handling in ListenPacket to
avoid treading on someone's socket descriptors which are recycled by
descriptor duplication. The issue happens when calling ListenPacket for
non-privileged ICMP endpoint concurrently.

Fixes golang/go#16969.

Change-Id: I4d7672535f5aeb2a0b71f8af2c7ba271a085f418
Reviewed-on: https://go-review.googlesource.com/28473
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>