mac80211: mesh: fix missing unlock on error in table_path_del()

spin_lock_bh() is used in table_path_del() but rcu_read_unlock()
is used for unlocking. Fix it by using spin_unlock_bh() instead
of rcu_read_unlock() in the error handling case.

Fixes: b4c3fbe6

 ("mac80211: Use linked list instead of rhashtable walk for mesh tables")
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>