net/ipv6: Updating ll reserve should be done for all IPv6 packet.

Doing it only in net_context, prevented to do it once NS succesfully
finished. This generated an error in 15.4, where pending data had wrong
ll reserve size.

Change-Id: I0f917fb76171457e5dff2c29e44edb8f00662150
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>