Use different name for the same constraint

In DB2, a constraint-name must not identify a constraint
that was already specified within the same CREATE TABLE
statement. The current CREATE TABLE statement uses the same
constraint name 'ipv6_modes' for both ipv6_ra_mode and
ipv6_address_mode column. This change tries to use different
names.

Change-Id: Id4d82fb7e0e570a843e28856e531e25578a4351a
Closes-Bug: #1294568
Merge "Use different name for the same constraint"