ioam6: improve checks on user data

This patch improves two checks on user data.

The first one prevents bit 23 from being set, as specified by RFC 9197
(Sec 4.4.1):

  Bit 23    Reserved; MUST be set to zero upon transmission and be
            ignored upon receipt.  This bit is reserved to allow for
            future extensions of the IOAM Trace-Type bit field.

The second one checks that the tunnel destination address !=
IPV6_ADDR_ANY, just like we already do for the tunnel source address.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://patch.msgid.link/20240830191919.51439-1-justin.iurman@uliege.be


Signed-off-by: Jakub Kicinski <kuba@kernel.org>