selftests: forwarding: tc_actions: Use ncat instead of nc

The test relies on 'nc' being the netcat version from the nmap project.
While this seems to be the case on Fedora, it is not the case on Ubuntu,
resulting in failures such as [1].

Fix by explicitly using the 'ncat' utility from the nmap project and the
skip the test in case it is not installed.

[1]
 # timeout set to 0
 # selftests: net/forwarding: tc_actions.sh
 # TEST: gact drop and ok (skip_hw)                                    [ OK ]
 # TEST: mirred egress flower redirect (skip_hw)                       [ OK ]
 # TEST: mirred egress flower mirror (skip_hw)                         [ OK ]
 # TEST: mirred egress matchall mirror (skip_hw)                       [ OK ]
 # TEST: mirred_egress_to_ingress (skip_hw)                            [ OK ]
 # nc: invalid option -- '-'
 # usage: nc [-46CDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
 #         [-m minttl] [-O length] [-P proxy_username] [-p source_port]
 #         [-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit]
 #         [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
 #         [destination] [port]
 # nc: invalid option -- '-'
 # usage: nc [-46CDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
 #         [-m minttl] [-O length] [-P proxy_username] [-p source_port]
 #         [-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit]
 #         [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
 #         [destination] [port]
 # TEST: mirred_egress_to_ingress_tcp (skip_hw)                        [FAIL]
 #       server output check failed
 # INFO: Could not test offloaded functionality
 not ok 80 selftests: net/forwarding: tc_actions.sh # exit=1

Fixes: ca22da2f ("act_mirred: use the backlog for nested calls to mirred ingress")
Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Closes: https://lore.kernel.org/netdev/adc5e40d-d040-a65e-eb26-edf47dac5b02@alu.unizg.hr/Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20230808141503.4060661-12-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/tc_actions.sh

@@ -9,6 +9,8 @@ NUM_NETIFS=4
 source tc_common.sh
 source lib.sh
 
+require_command ncat
+
 tcflags="skip_hw"
 
 h1_create()
@@ -220,9 +222,9 @@ mirred_egress_to_ingress_tcp_test()
 		ip_proto icmp \
 			action drop
 
-	ip vrf exec v$h1 nc --recv-only -w10 -l -p 12345 -o $mirred_e2i_tf2  &
+	ip vrf exec v$h1 ncat --recv-only -w10 -l -p 12345 -o $mirred_e2i_tf2 &
 	local rpid=$!
-	ip vrf exec v$h1 nc -w1 --send-only 192.0.2.2 12345 <$mirred_e2i_tf1
+	ip vrf exec v$h1 ncat -w1 --send-only 192.0.2.2 12345 <$mirred_e2i_tf1
 	wait -n $rpid
 	cmp -s $mirred_e2i_tf1 $mirred_e2i_tf2
 	check_err $? "server output check failed"