xfrm: Allow inserting policies with matching mark and different priorities

We currently can not insert policies with mark and mask
such that some flows would be matched from both policies.
We make this possible when the priority of these policies
are different. If both policies match a flow, the one with
the higher priority is used.

Reported-by: Emmanuel Thierry <emmanuel.thierry@telecom-bretagne.eu>
Reported-by: Romain Kuntz <r.kuntz@ipflavors.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>