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

The only exception is `cmd/vdev_id/vdev_id` which might be a subject of
refactoring (see #12084)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes #12912

cmd/zpool/zpool.d/iostat

@@ -27,6 +27,7 @@ elif [ "$script" = "iostat-10s" ] ; then
 	brief="yes"
 fi
 
+# shellcheck disable=SC2154
 if [ -f "$VDEV_UPATH" ] ; then
 	# We're a file-based vdev, iostat doesn't work on us.  Do nothing.
 	exit
@@ -67,7 +68,7 @@ i=0
 for col in $cols ; do
 	i=$((i+1))
 	# Skip the first column since it's just the device name
-	if [ $i -eq 1 ]; then
+	if [ "$i" -eq 1 ]; then
 		continue
 	fi