sparsemem: make SPARSEMEM_VMEMMAP selectable

SPARSEMEM_VMEMMAP needs to be a selectable config option to support
building the kernel both with and without sparsemem vmemmap support. This
selection is desirable for platforms which could be configured one way for
platform specific builds and the other for multi-platform builds.

Signed-off-by: Miguel Botón <[e-mail]>
Signed-off-by: Geoff Levand <[e-mail]>
Acked-by: Yasunori Goto <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Andy Whitcroft <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/Kconfig

···
 	def_bool y
 	depends on SPARSEMEM && !SPARSEMEM_STATIC
 
-#
-# SPARSEMEM_VMEMMAP uses a virtually mapped mem_map to optimise pfn_to_page
-# and page_to_pfn.  The most efficient option where kernel virtual space is
-# not under pressure.
-#
 config SPARSEMEM_VMEMMAP_ENABLE
 	def_bool n
 
 config SPARSEMEM_VMEMMAP
-	bool
-	depends on SPARSEMEM
-	default y if (SPARSEMEM_VMEMMAP_ENABLE)
+	bool "Sparse Memory virtual memmap"
+	depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
+	default y
+	help
+	 SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
+	 pfn_to_page and page_to_pfn operations.  This is the most
+	 efficient option when sufficient kernel resources are available.
 
 # eventually, we can have this option just 'select SPARSEMEM'
 config MEMORY_HOTPLUG