fix unexpected MOUNTPATH change when $KSU = false

[255doesnotexist]

$KSU && MOUNTPATH=$(dirname $MODPATH) || MOUNTPATH="$(magisk --path)/.magisk/modules"

will accidentally change MOUNTPATH if $KSU = false.

$KSU = false make the whole expression '$KSU && MOUNTPATH=$(dirname $MODPATH)' = false.

then shell will execute 'MOUNTPATH="$(magisk --path)/.magisk/modules" '

that is accidentally changed Magisk users' MOUNTPATH

so add the "$KSU &&" after the "||" is absolutely patch to reach maintainers original meanings

(as the "&&"'s priority higher than "||")

[255doesnotexist]

not work on other devices. i should take it back and think twice.