zram-swap: enable swap discard

zram-swap: enable swap discard

Zram block devices have supported trim/discard for over six years, let's
enable it. This allows the zram device to actually free up allocated memory
when it's marked as unused in the filesystem metadata, as explained in more
detail in the original commit message [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/block/zram/zram_drv.c?h=linux-4.14.y&id=f4659d8e620d08bd1a84a8aec5d2f5294a242764

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>