ipv6: ioam: Distinguish input and output for hop-limit

This patch anticipates the support for the IOAM insertion inside in-transit
packets, by making a difference between input and output in order to determine
the right value for its hop-limit (inherited from the IPv6 hop-limit).

Input case: happens before ip6_forward, the IPv6 hop-limit is not decremented
yet -> decrement the IOAM hop-limit to reflect the new hop inside the trace.

Output case: happens after ip6_forward, the IPv6 hop-limit has already been
decremented -> keep the same value for the IOAM hop-limit.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David S. Miller <davem@davemloft.net>