[XFRM]: Clearing xfrm_policy_count[] to zero during flush is incorrect.

When we flush policies, we do a type match so we might not
actually delete all policies matching a certain direction.

So keep track of how many policies we actually kill and
subtract that number from xfrm_policy_count[dir] at the
end.

Based upon a patch by Masahide NAKAMURA.

Signed-off-by: David S. Miller <davem@davemloft.net>