[XFRM] netlink: Establish an attribute policy

Adds a policy defining the minimal payload lengths for all the attributes
allowing for most attribute validation checks to be removed from in
the middle of the code path. Makes updates more consistent as many format
errors are recognised earlier, before any changes have been attempted.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>