tipc: correct usage of spin_lock() vs spin_lock_bh()

I commit e099e86c
("tipc: add node_lock protection to link lookup function")
we are calling spin_lock(&node->lock) directly instead of indirectly
via the tipc_node_lock(node) function. However, tipc_node_lock() is
using spin_lock_bh(), not spin_lock(), something leading to
unbalanced usage in one place, and a smatch warning.

We fix this by consistently using tipc_node_lock()/unlock() in
in the places touched by the mentioned commit.

Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>