tipc: fix shift wrapping bug in map_get()

There is a shift wrapping bug in this code so anything thing above
31 will return false.

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>