ovs-rcu: Name the ovsrcu_postpone_thread to 'urcu'.

The ovs-rcu module adds a new thread for checking the grace period.
Since the thread name is not set, it will inherit the name of the
thread that creates it.  This makes the 'top' output quite confusing.

This commit names the thread to 'urcu' for clarity.

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