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: 136a1b43

 ("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>