xfrm: security: iterate all, not inexact lists

currently all non-socket policies are either hashed in the dst table,
or placed on the 'inexact list'.  When flushing, we first walk the
table, then the (per-direction) inexact lists.

When we try and get rid of the inexact lists to having "n" inexact
lists (e.g. per-af inexact lists, or sorted into a tree), this walk
would become more complicated.

Simplify this: walk the 'all' list and skip socket policies during
traversal so we don't need to handle exact and inexact policies
separately anymore.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>