mktmpdir: fixed sort key

The sort key field value has been off-by-one apparently,
"6" corresponds to "capacity" field of df -PT output
while the "available" one was meant:

Filesystem     Type  1024-blocks     Used Available Capacity Mounted on
/dev/sda9      ext4     15350768 12799972   1747980      88% /
tmpfs          tmpfs     1967204    27396   1939808       2% /tmp

1              2               3        4         5       6

This lapse has been five years old, looks like it's only
been masked by lower available space limit *and* filesystem
ordering by type so far.  It didn't hit me even now, rather
inspected the code while implementing volumes-profile-starterkit...

bin/mktmpdir

@@ -45,7 +45,7 @@ choose_tmpdir() {
 		echo -n "$i "
 		df -PT "$i" | tail -1
 	done \
-	| sort -unk6 \
+	| sort -unk5 \
 	| while read dir dev fstype size used free percent mnt; do
 		check_options "$dev" "$mnt" || continue
 		[ "$free" -gt "$MINSIZE" ] || continue