selftests: net: don't access /dev/stdout in pmtu.sh

When running the pmtu.sh via the kselftest infra, accessing
/dev/stdout gives unexpected results:
  # dd: failed to open '/dev/stdout': Device or resource busy
  # TEST: IPv4, bridged vxlan4: PMTU exceptions                         [FAIL]

Let dd use directly the standard output to fix the above:
  # TEST: IPv4, bridged vxlan4: PMTU exceptions - nexthop objects       [ OK ]

Fixes: 136a1b4 ("selftests: net: test vxlan pmtu exceptions with tcp")
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/23d7592c5d77d75cff9b34f15c227f92e911c2ae.1706635101.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/pmtu.sh

@@ -1339,7 +1339,7 @@ test_pmtu_ipvX_over_bridged_vxlanY_or_geneveY_exception() {
 
 		sleep 1
 
-		dd if=/dev/zero of=/dev/stdout status=none bs=1M count=1 | ${target} socat -T 3 -u STDIN $TCPDST,connect-timeout=3
+		dd if=/dev/zero status=none bs=1M count=1 | ${target} socat -T 3 -u STDIN $TCPDST,connect-timeout=3
 
 		size=$(du -sb $tmpoutfile)
 		size=${size%%/tmp/*}