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

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

Here is the test result after conversion.

 ]# ./fib_rule_tests.sh

     TEST: rule6 check: oif redirect to table                  [ OK ]

     ...

     TEST: rule4 dsfield tcp connect (dsfield 0x07)            [ OK ]

 Tests passed:  66
 Tests failed:   0

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-12-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>