selftests: mptcp: netlink: drop disable=SC2086

Now there are only a few of variables are not using double quotes.
Modifying them, then "shellcheck disable=SC2086" can be dropped.

Signed-off-by: Geliang Tang <[e-mail]>
Reviewed-by: Matthieu Baerts (NGI0) <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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"
 
 ret=0
···
 while getopts "$optstring" option;do
 	case "$option" in
 	"h")
-		usage $0
+		usage "$0"
 		exit ${KSFT_PASS}
 		;;
 	"i")
 		mptcp_lib_set_ip_mptcp
 		;;
 	"?")
-		usage $0
+		usage "$0"
 		exit ${KSFT_FAIL}
 		;;
 	esac
···
 #shellcheck disable=SC2317
 cleanup()
 {
-	rm -f $err
+	rm -f "${err}"
 	mptcp_lib_ns_exit "${ns1}"
 }