staging: zcache: sync to kernel.org

staging: zcache: fix cleancache race condition with shrinker
@ 6d7d9798ad5c97ee4e911dd070dc12dc5ae55bd0

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

Conflicts:

	drivers/staging/zcache/Kconfig
	drivers/staging/zcache/zcache-main.c
zcache-main: enable zcache by default

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