xfrm: add option to hide keys in state output

xfrm: add option to hide keys in state output

ip xfrm state show currently dumps keys unconditionally. This limits its
use in logging, as security information can be leaked.

This patch adds a nokeys option to ip xfrm ( state show | monitor ), which
prevents the printing of keys. This allows ip xfrm state show to be used
in logging without exposing keys.

Signed-off-by: Benedict Wong <benedictwong@google.com>
Signed-off-by: David Ahern <dsahern@gmail.com>