selftests/net: convert xfrm_policy.sh to run it in unique namespace

Here is the test result after conversion.

 # ./xfrm_policy.sh
 PASS: policy before exception matches
 PASS: ping to .254 bypassed ipsec tunnel (exceptions)
 PASS: direct policy matches (exceptions)
 PASS: policy matches (exceptions)
 PASS: ping to .254 bypassed ipsec tunnel (exceptions and block policies)
 PASS: direct policy matches (exceptions and block policies)
 PASS: policy matches (exceptions and block policies)
 PASS: ping to .254 bypassed ipsec tunnel (exceptions and block policies after hresh changes)
 PASS: direct policy matches (exceptions and block policies after hresh changes)
 PASS: policy matches (exceptions and block policies after hresh changes)
 PASS: ping to .254 bypassed ipsec tunnel (exceptions and block policies after hthresh change in ns3)
 PASS: direct policy matches (exceptions and block policies after hthresh change in ns3)
 PASS: policy matches (exceptions and block policies after hthresh change in ns3)
 PASS: ping to .254 bypassed ipsec tunnel (exceptions and block policies after htresh change to normal)
 PASS: direct policy matches (exceptions and block policies after htresh change to normal)
 PASS: policy matches (exceptions and block policies after htresh change to normal)
 PASS: policies with repeated htresh change

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>