Prepare to set and dump the tunnel flags.

This code won't see any of these flags yet, as these flags
aren't allowed by the NLA_POLICY_MASK, and the functions
doesn't get called with encap set to true yet.
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Tested-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Davide Caratti <dcaratti@redhat.com>
Link: https://patch.msgid.link/20240713021911.1631517-9-ast@fiberby.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>