tipc: Hide internal details of node table implementation

Relocates information about the size of TIPC's node table index and
its associated hash function, since only node subsystem routines need
to have access to this information.

Note that these changes are essentially cosmetic in nature, and have
no impact on the actual operation of TIPC.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>