selftests: net: fix available tunnels detection

The pmtu.sh test tries to detect the tunnel protocols available
in the running kernel and properly skip the unsupported cases.

In a few more complex setup, such detection is unsuccessful, as
the script currently ignores some intermediate error code at
setup time.

Before:
  # which: no nettest in (/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin)
  # TEST: vti6: PMTU exceptions (ESP-in-UDP)                            [FAIL]
  #   PMTU exception wasn't created after creating tunnel exceeding link layer MTU
  # ./pmtu.sh: line 931: kill: (7543) - No such process
  # ./pmtu.sh: line 931: kill: (7544) - No such process

After:
  #   xfrm4 not supported
  # TEST: vti4: PMTU exceptions                                         [SKIP]

Fixes: ece1278a ("selftests: net: add ESP-in-UDP PMTU test")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/cab10e75fda618e6fff8c595b632f47db58b9309.1706635101.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/pmtu.sh

@@ -707,23 +707,23 @@ setup_xfrm6() {
 }
 
 setup_xfrm4udp() {
-	setup_xfrm 4 ${veth4_a_addr} ${veth4_b_addr} "encap espinudp 4500 4500 0.0.0.0"
-	setup_nettest_xfrm 4 4500
+	setup_xfrm 4 ${veth4_a_addr} ${veth4_b_addr} "encap espinudp 4500 4500 0.0.0.0" && \
+		setup_nettest_xfrm 4 4500
 }
 
 setup_xfrm6udp() {
-	setup_xfrm 6 ${veth6_a_addr} ${veth6_b_addr} "encap espinudp 4500 4500 0.0.0.0"
-	setup_nettest_xfrm 6 4500
+	setup_xfrm 6 ${veth6_a_addr} ${veth6_b_addr} "encap espinudp 4500 4500 0.0.0.0" && \
+		setup_nettest_xfrm 6 4500
 }
 
 setup_xfrm4udprouted() {
-	setup_xfrm 4 ${prefix4}.${a_r1}.1 ${prefix4}.${b_r1}.1 "encap espinudp 4500 4500 0.0.0.0"
-	setup_nettest_xfrm 4 4500
+	setup_xfrm 4 ${prefix4}.${a_r1}.1 ${prefix4}.${b_r1}.1 "encap espinudp 4500 4500 0.0.0.0" && \
+		setup_nettest_xfrm 4 4500
 }
 
 setup_xfrm6udprouted() {
-	setup_xfrm 6 ${prefix6}:${a_r1}::1 ${prefix6}:${b_r1}::1 "encap espinudp 4500 4500 0.0.0.0"
-	setup_nettest_xfrm 6 4500
+	setup_xfrm 6 ${prefix6}:${a_r1}::1 ${prefix6}:${b_r1}::1 "encap espinudp 4500 4500 0.0.0.0" && \
+		setup_nettest_xfrm 6 4500
 }
 
 setup_routing_old() {