tipc: Add network address mask helper routines

Introduces a pair of helper routines that convert the network address
for a TIPC node into the network address for its cluster or zone.

This is a cosmetic change designed to avoid future errors caused by
the incorrect use of address bitmasks, and does not alter the existing
operation of TIPC.

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