tests: Whitelist messages about RCU blocking in the testsuite.

In production these may indicate a bug, but in the testsuite they probably
just indicate a time-warp.

Reported-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>
ovs-rcu: Log the name of the main thread as "main" instead of "".

The main thread has the empty string as its name, but that's not a good
log string.

Without this patch we can get log message like
    blocked 1000 ms waiting for  to quiesce
from ovsrcu_synchronize().

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>