selftests: mptcp: diag: fix shellcheck warnings

shellcheck recently helped to prevent issues. It is then good to fix the
other harmless issues in order to spot "real" ones later.

Here, two categories of warnings are now ignored:

- SC2317: Command appears to be unreachable. The cleanup() function is
  invoked indirectly via the EXIT trap.

- SC2086: Double quote to prevent globbing and word splitting. This is
  recommended, but the current usage is correct and there is no need to
  do all these modifications to be compliant with this rule.

For the modifications:

  - SC2034: ksft_skip appears unused.
  - SC2046: Quote '$(get_msk_inuse)' to prevent word splitting.
  - SC2006: Use $(...) notation instead of legacy backticks `...`.

Now this script is shellcheck (0.9.0) compliant. We can easily spot new
issues.
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-7-bc79e6e5e6a0@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/diag.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+# Double quotes to prevent globbing and word splitting is recommended in new
+# code but we accept it, especially because there were too many before having
+# address all other issues detected by shellcheck.
+#shellcheck disable=SC2086
+
 . "$(dirname "${0}")/mptcp_lib.sh"
 
 ns=""
-ksft_skip=4
 test_cnt=1
 timeout_poll=30
 timeout_test=$((timeout_poll * 2 + 1))
@@ -24,6 +28,8 @@ flush_pids()
 	done
 }
 
+# This function is used in the cleanup trap
+#shellcheck disable=SC2317
 cleanup()
 {
 	ip netns pids "${ns}" | xargs --no-run-if-empty kill -SIGKILL &>/dev/null
@@ -174,7 +180,7 @@ chk_msk_inuse()
 	expected=$((expected + listen_nr))
 
 	for _ in $(seq 10); do
-		if [ $(get_msk_inuse) -eq $expected ];then
+		if [ "$(get_msk_inuse)" -eq $expected ]; then
 			break
 		fi
 		sleep 0.1
@@ -260,7 +266,7 @@ chk_msk_inuse 0 "1->0"
 chk_msk_cestab 0 "1->0"
 
 NR_CLIENTS=100
-for I in `seq 1 $NR_CLIENTS`; do
+for I in $(seq 1 $NR_CLIENTS); do
 	echo "a" | \
 		timeout ${timeout_test} \
 			ip netns exec $ns \
@@ -269,7 +275,7 @@ for I in `seq 1 $NR_CLIENTS`; do
 done
 mptcp_lib_wait_local_port_listen $ns $((NR_CLIENTS + 10001))
 
-for I in `seq 1 $NR_CLIENTS`; do
+for I in $(seq 1 $NR_CLIENTS); do
 	echo "b" | \
 		timeout ${timeout_test} \
 			ip netns exec $ns \