drivers: staging: zcache: fix compile error

Because 'ramster_debugfs_init' is not defined if !CONFIG_DEBUG_FS, there is
compile error:

$ make drivers/staging/zcache/
staging/zcache/ramster/ramster.c: In function ‘ramster_init’:
staging/zcache/ramster/ramster.c:981:2: error: implicit declaration of
function ‘ramster_debugfs_init’ [-Werror=implicit-function-declaration]

This patch fix it and reduce some #ifdef CONFIG_DEBUG_FS in .c files the same
way.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>