net: ipv6: ioam6: use consistent dst names

Be consistent and use the same terminology as other lwt users: orig_dst
is the dst_entry before the transformation, while dst is either the
dst_entry in the cache or the dst_entry after the transformation

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://patch.msgid.link/20250415112554.23823-2-justin.iurman@uliege.be


Signed-off-by: Paolo Abeni <pabeni@redhat.com>