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

tools/testing/selftests/net/fdb_flush.sh

@@ -5,6 +5,8 @@
 # Check that flush works as expected with all the supported arguments and verify
 # some combinations of arguments.
 
+source lib.sh
+
 FLUSH_BY_STATE_TESTS="
 	vxlan_test_flush_by_permanent
 	vxlan_test_flush_by_nopermanent
@@ -739,10 +741,9 @@ bridge_vxlan_test_flush()
 
 setup()
 {
-	IP="ip -netns ns1"
-	BRIDGE="bridge -netns ns1"
-
-	ip netns add ns1
+	setup_ns NS
+	IP="ip -netns ${NS}"
+	BRIDGE="bridge -netns ${NS}"
 
 	$IP link add name vx10 type vxlan id 1000 dstport "$VXPORT"
 	$IP link add name vx20 type vxlan id 2000 dstport "$VXPORT"
@@ -759,7 +760,7 @@ cleanup()
 	$IP link del dev vx20
 	$IP link del dev vx10
 
-	ip netns del ns1
+	cleanup_ns ${NS}
 }
 
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