net: ipv6: Fix handling of RA 6CO options

As per RFC 6775 4.2, context length holds number of leading bits
in the Context Prefix field that are valid. Rest are not valid
for compression.

Change-Id: Id21cc2d7a5d42980cf9295f85e75c4869ff6cb99
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>