init/initramfs: fix warning with CONFIG_BLK_DEV_RAM=n

init/initramfs.c:520: warning: 'clean_rootfs' defined but not used

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>