tipc: avoid double lock 'spin_lock:&seq->lock'

The commit fb9962f3

 ("tipc: ensure all name sequences are properly
protected with its lock") involves below errors:

net/tipc/name_table.c:980 tipc_purge_publications() error: double lock 'spin_lock:&seq->lock'

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>