99memstrack: hook script should not call exit

99memstrack: hook script should not call exit

With memstrack module, rd.break may not work because the hook scripts
are sourced, not executed, so the exit call will make pre-pivot queue
exit early. See 98dracut-systemd/dracut-pre-pivot.sh, everything after
"source_hook cleanup" will be ignored.

Replace with return instead.

Credits go to Lukas Nykryn <lnykryn@redhat.com> who helped discover and
debug this issue.

Signed-off-by: Kairui Song <kasong@redhat.com>