selftests/net: change shebang to bash to support "source"

The patch set [1] added a general lib.sh in net selftests, and converted
several test scripts to source the lib.sh.

unicast_extensions.sh (converted in [1]) and pmtu.sh (converted in [2])
have a /bin/sh shebang which may point to various shells in different
distributions, but "source" is only available in some of them. For
example, "source" is a built-it function in bash, but it cannot be
used in dash.

Refer to other scripts that were converted together, simply change the
shebang to bash to fix the following issues when the default /bin/sh
points to other shells.

not ok 51 selftests: net: unicast_extensions.sh # exit=1

v1 -> v2:
  - Fix pmtu.sh which has the same issue as unicast_extensions.sh,
    suggested by Hangbin
  - Change the style of the "source" line to be consistent with other
    tests, suggested by Hangbin

Link: https://lore.kernel.org/all/20231202020110.362433-1-liuhangbin@gmail.com/ [1]
Link: https://lore.kernel.org/all/20231219094856.1740079-1-liuhangbin@gmail.com/ [2]
Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: 378f082e ("selftests/net: convert pmtu.sh to run it in unique namespace")
Fixes: 0f4765d0 ("selftests/net: convert unicast_extensions.sh to run it in unique namespace")
Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20231229131931.3961150-1-yujie.liu@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/pmtu.sh

-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 #
 # Check that route PMTU values match expectations, and that initial device MTU
@@ -198,7 +198,7 @@
 # - pmtu_ipv6_route_change
 #	Same as above but with IPv6
 
-source ./lib.sh
+source lib.sh
 
 PAUSE_ON_FAIL=no
 VERBOSE=0

tools/testing/selftests/net/unicast_extensions.sh

-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 #
 # By Seth Schoen (c) 2021, for the IPv4 Unicast Extensions Project
@@ -28,7 +28,7 @@
 # These tests provide an easy way to flip the expected result of any
 # of these behaviors for testing kernel patches that change them.
 
-source ./lib.sh
+source lib.sh
 
 # nettest can be run from PATH or from same directory as this selftest
 if ! which nettest >/dev/null; then