zRam : add LZ4 compression option as alternative to LZO or Snappy compression to use as zRam compression algorithm

- LZ4 algorithm addition (psndna88)
- integrating LZ4 keeping Snappy as an option (Yank555.lu)

Conflicts:
	drivers/staging/zram/Kconfig
	drivers/staging/zram/zram_drv.c