lib/hmap: Remove the memory fence from hmap_is_empty().

The fence made classifier_lookup() slower.  Access to a size_t 'n' is
safe without synchonizing, and if racing with writers matters,
additional syncronization primitives are used anyway.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
ovs-rcu: Call ovsrcu_init() in ovsrcu_quiesce().

This commit fixes a bug introduced by 0f2ea848(ovs-rcu: New library.).
It is possible that ovsrcu_quiesce() is called before ovsrcu_init().
So, it is necessary to call ovsrcu_init() in ovsrcu_quiesce().

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