net: ipv6: Fix IPv6 prefix comparision

If the prefix length % 8 is not 0, then the remaining
bit length was calculated incorrectly and the prefixes
were claimed to match even though they might not be the
same.

Adding a test cases for testing this properly.

Coverity-CID: 157591

Change-Id: I9cb5a73d5cc211ec183176400fa5e2dfd209e2da
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>