Here are two simple modifications, not really fixing anything for us but
fixing issues for those using BusyBox like LKFT.

At least the first patch fixes a bug they have. They might switch to a
different version of cmp at some points but it doesn't seem easy. So
probably best to have this patch flagged as a fix to have it backported
in order to detect when other mptcp_join.sh subtests are failing.

The second patch fixes ShellCheck warnings: it was reporting issues for
most lines of the script. Best to fix these warnings to easily spot new
warnings that could be linked to real issues. But no need to backport
this one I suppose.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
---
Changes in v3:
- patch 1/2 use a tmp file and delete it in the cleanup phase just in
  case the operation is stopped between 'head > file' and 'mv file'.
- Link to v2: https://lore.kernel.org/r/20230404-mptcp-selftests-join-busybox-shellcheck-v2-0-9113fa4d2ce2@tessares.net

Changes in v2:
- Patch 1/2 now shrink files instead of doing something useless
- Link to v1: https://lore.kernel.org/r/20230404-mptcp-selftests-join-busybox-shellcheck-v1-0-65892428a951@tessares.net

---
Matthieu Baerts (2):
      selftests: mptcp: join: avoid using 'cmp --bytes'
      selftests: mptcp: join: fix ShellCheck warnings

 tools/testing/selftests/net/mptcp/mptcp_join.sh | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)
---
base-commit: 7abc2bc2d1a8d495956f75440f45088d7d118f9a
change-id: 20230404-mptcp-selftests-join-busybox-shellcheck-ec239f8d4e8f

Best regards,
-- 
Matthieu Baerts <matthieu.baerts@tessares.net>