Merge branch 'xfrm: fix uapi for the default policy'

Nicolas Dichtel says:

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This feature has just been merged after the last release, thus it's still
time to fix the uapi.
As stated in the thread, the uapi is based on some magic values (from the
userland POV).
Here is a proposal to simplify this uapi and make it clear how to use it.
The other problem was the notification: changing the default policy may
radically change the packets flows.

v2 -> v3: rebase on top of ipsec tree

v1 -> v2: fix warnings reported by the kernel test robot
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Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>