xfrm_policy: Add filter option for socket policies

xfrm_policy: Add filter option for socket policies

Listing policies on systems with a lot of socket policies can be
confusing due to the number of returned polices. Even if socket polices
are not of interest, they cannot be filtered. This patch adds an option
to filter all socket policies from the output.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>