90crypt/module-setup.sh: fix force on multiple lines

The first line in crypttab with a "force" option causes all subsequent
lines to be included as if they also had it set because the variable
used to track it is not reset between loop iterations.  So fix that by
just setting it to empty before the check for the force option.

modules.d/90crypt/module-setup.sh

@@ -113,6 +113,7 @@ install() {
             set -- ${luksoptions}
             IFS="${OLD_IFS}"
 
+            forceentry=""
             while [ $# -gt 0 ]; do
                 case $1 in
                     force)