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

Here is the test result after conversion.
 # ./fdb_flush.sh
 TEST: vx10: Expected 5 FDB entries, got 5                           [ OK ]
 TEST: vx20: Expected 5 FDB entries, got 5                           [ OK ]
 ...
 TEST: vx10: Expected 5 FDB entries, got 5                           [ OK ]
 TEST: Test entries with dst 192.0.2.1                               [ OK ]

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-14-liuhangbin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>