xfrm: make user policy API complete

>From a userland POV, this API was based on some magic values:
 - dirmask and action were bitfields but meaning of bits
   (XFRM_POL_DEFAULT_*) are not exported;
 - action is confusing, if a bit is set, does it mean drop or accept?

Let's try to simplify this uapi by using explicit field and macros.

Fixes: 2d151d39

 ("xfrm: Add possibility to set the default to block if we have no policy")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>