dumpstate: set backlog to zero

Set backlog to 0 to make sure that queue size will be minimum.
In Linux, because the minimum queue will be 1, connect() will be blocked
if the other clients already called connect() and the connection request
was not accepted.

Bug: 123306389
Bug: 135575100
Test: execute bugreport in parallel

Change-Id: Iee94de88deb82746818b729387ed9cd39e9b2f99
Signed-off-by: Kansho Nishida <kansho@google.com>