staging: zcache: make zcache builtin only

zcache cannot currently be loaded as a module.  However
the Kconfig allows it to be built as a module; something that
the user probably does not intend since the module is not
loadable.

This patch switches zcache from a tristate to a bool in the Kconfig
Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/Kconfig

 config ZCACHE
-	tristate "Dynamic compression of swap pages and clean pagecache pages"
+	bool "Dynamic compression of swap pages and clean pagecache pages"
 	# X86 dependency is because zsmalloc uses non-portable pte/tlb
 	# functions
 	depends on (CLEANCACHE || FRONTSWAP) && CRYPTO && X86