net: ipv6: Allow user to tweak packet hop limit value

The default hop limit is defined in struct net_if. It is possible
that user might want to tweak it for each network packet. For this
purpose, a net_nbuf_set_ipv6_hop_limit(buf) function is created.

Change-Id: I7568330358f80f0f5007d6d3c411c120b043c04f
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>