Add index generation for IPv6 rules for DVR

Add index generation for IPv6 rules for DVR

For IPv6 support with DVR the index used for rule priority and
route table needs to be generated such that the index is 32 bits
or less but greater than the system generated rule entries.

For IPv4 the numeric value of the network is used as the index.
For IPv6 the 30 bit xor-folded crc32 of the cidr is used.
Values smaller than system generated entries are extended into
the range freed up because of xor-folding.
For code modularity, index generation is wrapped into a helper
method.

Partial-bug: #1376325

Change-Id: I4bcde80257ef5fe7c744086019841cdef5e3c60c