zram: support zram_writeback

This patch supports zram_writeback enabled by fstab in two ways.

e.g.,
/dev/block/zram0 none swap defaults
    zramsize=1073741824,max_comp_streams=8,zram_loopback_path=/data/unencrypted/zram_swap

==> loopback size is 512MB by default.

Or,
/dev/block/zram0 none swap defaults
    zramsize=1073741824,max_comp_streams=8,zram_loopback_path=/data/unencrypted/zram_swap,zram_loopback_size=1G

==> loopback size can be specified by "zram_loopback_size=%s" with "GB" or "MB".

Or,
/dev/block/zram0 none swap defaults
    zramsize=1073741824,max_comp_streams=8,zram_backing_dev_path=/dev/block/partition

Bug: 74582279
Bug: 122659265
Change-Id: I66a2e6953b4743a34cf732dd0f5b5256c901f247
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>