block: zram: Add support for dynamic compressor switch

block: zram: Add support for dynamic compressor switch

Orginally, the compression algorithm cannot be changed until
resetting it if zram is once initialized by setting disksize. Since
this, however, zram can have multiple compressor and switch them
dynamically.

With this, zram uses the algorithm which is fast but has low
compression ratio at first, and can change to the one that is slightly
slower but having higher ratio when the target get bothered by low
memory.

Change-Id: I3fd817e299a76284b8b28e318a4822107e6a5f6d
Signed-off-by: Dongwoo Lee <dwoo08.lee@samsung.com>