[XFRM] netlink: Use nlmsg_parse() to parse attributes

Uses nlmsg_parse() to parse the attributes. This actually changes
behaviour as unknown attributes (type > MAXTYPE) no longer cause
an error. Instead unknown attributes will be ignored henceforth
to keep older kernels compatible with more recent userspace tools.

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