selftests: mptcp: pm netlink: 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.
  - SC2154: optstring is referenced but not assigned.
  - 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-v2-10-bc79e6e5e6a0@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/pm_netlink.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"
 
-ksft_skip=4
 ret=0
 
 usage() {
 	echo "Usage: $0 [ -h ]"
 }
 
-
+optstring=h
 while getopts "$optstring" option;do
 	case "$option" in
 	"h")
@@ -27,6 +31,8 @@ done
 ns1=""
 err=$(mktemp)
 
+# This function is used in the cleanup trap
+#shellcheck disable=SC2317
 cleanup()
 {
 	rm -f $err
@@ -91,14 +97,14 @@ check "ip netns exec $ns1 ./pm_nl_ctl get 4" "" "duplicate addr"
 ip netns exec $ns1 ./pm_nl_ctl add 10.0.1.4 flags signal
 check "ip netns exec $ns1 ./pm_nl_ctl get 4" "id 4 flags signal 10.0.1.4" "id addr increment"
 
-for i in `seq 5 9`; do
+for i in $(seq 5 9); do
 	ip netns exec $ns1 ./pm_nl_ctl add 10.0.1.$i flags signal >/dev/null 2>&1
 done
 check "ip netns exec $ns1 ./pm_nl_ctl get 9" "id 9 flags signal 10.0.1.9" "hard addr limit"
 check "ip netns exec $ns1 ./pm_nl_ctl get 10" "" "above hard addr limit"
 
 ip netns exec $ns1 ./pm_nl_ctl del 9
-for i in `seq 10 255`; do
+for i in $(seq 10 255); do
 	ip netns exec $ns1 ./pm_nl_ctl add 10.0.0.9 id $i
 	ip netns exec $ns1 ./pm_nl_ctl del $i
 done