fix plymouth-encrypt hook to pass the cryptargs

this fixes the issue with discard(aka TRIM) not working because --allow-discards was not passed to cryptsetup
(tested to work now)

example:
/etc/default/grub
GRUB_CMDLINE_LINUX="cryptdevice=/dev/disk/by-uuid/70c08890-417a-497d-b6ab-c0d0357a63e2:cryptManjaro:allow-discards"
the "allow-discards" at the end of the line was only passed(as --allow-discards to cryptsetup) when using the `encrypt` hook but not when using `plymouth-encrypt` hook. This prevented the use of `fstrim / -v` with errors saying it's not supported.
To test that discard is enabled in this case:
sudo dmsetup table|grep allow_discards
or
sudo cryptsetup status cryptManjaro|grep discards
or
sudo fstrim / -v
would not error.

I, not knowing what plymouth is(just a front-end), originally reported the bug here: https://bugs.freedesktop.org/show_bug.cgi?id=80146
My bad:)