mm: hide the FRONTSWAP Kconfig symbol

Select FRONTSWAP from ZSWAP instead of prompting for it.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Juergen Gross <[e-mail]>
Cc: Dan Streetman <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Konrad Rzeszutek Wilk <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Seth Jennings <[e-mail]>
Cc: Vitaly Wool <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/Kconfig

···
 	bool
 
 config FRONTSWAP
-	bool "Enable frontswap to cache swap pages if tmem is present"
-	depends on SWAP
-	help
-	  Frontswap is so named because it can be thought of as the opposite
-	  of a "backing" store for a swap device.  The data is stored into
-	  "transcendent memory", memory that is not directly accessible or
-	  addressable by the kernel and is of unknown and possibly
-	  time-varying size.  When space in transcendent memory is available,
-	  a significant swap I/O reduction may be achieved.  When none is
-	  available, all frontswap calls are reduced to a single pointer-
-	  compare-against-NULL resulting in a negligible performance hit
-	  and swap data is stored as normal on the matching swap device.
-
-	  If unsure, say Y to enable frontswap.
+	bool
 
 config CMA
 	bool "Contiguous Memory Allocator"
···
 
 config ZSWAP
 	bool "Compressed cache for swap pages (EXPERIMENTAL)"
-	depends on FRONTSWAP && CRYPTO=y
+	depends on SWAP && CRYPTO=y
+	select FRONTSWAP
 	select ZPOOL
 	help
 	  A lightweight compressed cache for swap pages.  It takes