[XFRM] Optimize policy dumping

This change optimizes the dumping of Security policies.

1) Before this change ..
speedopolis:~# time ./ip xf pol

real    0m22.274s
user    0m0.000s
sys     0m22.269s

2) Turn off sub-policies

speedopolis:~# ./ip xf pol

real    0m13.496s
user    0m0.000s
sys     0m13.493s

i suppose the above is to be expected

3) With this change ..
speedopolis:~# time ./ip x policy

real    0m7.901s
user    0m0.008s
sys     0m7.896s
1 file changed