Add ShellCheck's --enable=all inside scripts/

Strengthen static code analysis for shell scripts.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12914

scripts/zfs-helpers.sh

@@ -1,4 +1,5 @@
 #!/bin/sh
+# shellcheck disable=SC2154
 #
 # This script is designed to facilitate in-tree development and testing
 # by installing symlinks on your system which refer to in-tree helper
@@ -85,6 +86,8 @@ while getopts 'hdirv' OPTION; do
 		usage
 		exit
 		;;
+	*)
+		;;
 	esac
 done