zswap, frontswap, vnswap: extract and backport to Nexus series

Signed-off-by: Paul Reioux <reioux@gmail.com>

Conflicts:
	drivers/staging/Kconfig
	drivers/staging/Makefile
	include/linux/swap.h
	include/trace/events/vmscan.h
	mm/Makefile
	mm/internal.h
	mm/page_io.c
	mm/shmem.c
	mm/swapfile.c
	mm/vmscan.c