tipc: convert tipc_nametbl_size type from variable to macro

There is nothing changing this variable dynamically, so change
it to a macro to make that more obvious when reading the code.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>