xfrm: Allow Set Mark to be Updated Using UPDSA

Allow UPDSA to change "set mark" to permit
policy separation of packet routing decisions from
SA keying in systems that use mark-based routing.

The set mark, used as a routing and firewall mark
for outbound packets, is made update-able which
allows routing decisions to be handled independently
of keying/SA creation. To maintain consistency with
other optional attributes, the set mark is only
updated if sent with a non-zero value.

The per-SA lock and the xfrm_state_lock are taken in
that order to avoid a deadlock with
xfrm_timer_handler(), which also takes the locks in
that order.

Signed-off-by: Nathan Harold <nharold@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>