ipv6: ioam: move the check for undefined bits

The check for undefined bits in the trace type is moved from the input side to
the output side, while the input side is relaxed and now inserts default empty
values when an undefined bit is set.

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