tipc: potential shift wrapping bug in map_set()

"up_map" is a u64 type but we're not using the high 32 bits.

Fixes: 35c55c98 ('tipc: add neighbor monitoring framework')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>