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

Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Chester Kener <Cl3Kener@gmail.com>

Conflicts:
	mm/shmem.c
Fix

Signed-off-by: Chester Kener <Cl3Kener@gmail.com>