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

[ Upstream commit 05d92cb0

 ]

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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: e71e016a

 ("selftests: net: fix tcp listener handling in pmtu.sh")
Signed-off-by: Sasha Levin <sashal@kernel.org>