vlib: Handle race in thread barrier processing

vlib: Handle race in thread barrier processing

When CLIB_DEBUG is enabled, vlib_foreach_main macro asserts that
vlib_main it currently looks at is safely parked in barrier, by
checkling that vlib_main->parked_at_barrier is not 0.
Unfortunately, the check is racy - workers first increment the
atomic counter to indicate that they have reached the barrier
and _then_ set this_main->parked_at_barrier to 1. For the last
worker to suspend this opens the race - main thread is free
to execute and assert immediately after atomic counter has been
incremented, before worker gets to write to own parked_at_barrier.

Fix this by simply swapping the order of two operations.

Type: fix

Signed-off-by: Alexnader Kabaev <kan@FreeBSD.org>
Change-Id: Iae47abd6ca0be1c5413f5ecaefabc64cd7eac2ed