tipc: fix a missing rhashtable_walk_exit()

rhashtable_walk_exit() must be paired with rhashtable_walk_enter().

Fixes: 40f9f439 ("tipc: Fix tipc_sk_reinit race conditions")
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>