luci-mod-system/zram: replace deflate with zstd

Deflate is just too slow for zram. Replace it with zstd, which is much faster at
similar compression ratios.

As a side note, this is an ugly hack. We're hard-coding compression algorithms
which might not be available in the system. The availability should be parsed
from /sys/block/zram0/comp_algorithm.

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