mm/zsmalloc: make zsmalloc module-buildable

Now, we can build zsmalloc as module because unmap_kernel_range was
exported.

Signed-off-by: Minchan Kim <[e-mail]>
Cc: Nitin Gupta <[e-mail]>
Cc: Sergey Senozhatsky <[e-mail]>
Cc: Jerome Marchand <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/Kconfig

···
 	  See Documentation/vm/soft-dirty.txt for more details.
 
 config ZSMALLOC
-	bool "Memory allocator for compressed pages"
+	tristate "Memory allocator for compressed pages"
 	depends on MMU
 	default n
 	help