kernel: add another missing KASAN symbol

Enabling KERNEL_KASAN exposes yet another missing symbol. This did not
appear on bcm27xx but is appearing now on x86/64. Add a new kernel build
option for KASAN_VMALLOC to fix build on x86/64 with KERNEL_KASAN
enabled.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>

config/Config-kernel.in

@@ -170,6 +170,22 @@ config KERNEL_KASAN_EXTRA
 	  compile time.
 	  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 has more
 
+CONFIG KERNEL_KASAN_VMALLOC
+	bool "Back mappings in vmalloc space with real shadow memory"
+	depends on KERNEL_KASAN
+	  By default, the shadow region for vmalloc space is the read-only
+	  zero page. This means that KASAN cannot detect errors involving
+	  vmalloc space.
+
+	  Enabling this option will hook in to vmap/vmalloc and back those
+	  mappings with real shadow memory allocated on demand. This allows
+	  for KASAN to detect more sorts of errors (and to support vmapped
+	  stacks), but at the cost of higher memory usage.
+
+	  This option depends on HAVE_ARCH_KASAN_VMALLOC, but we can't
+	  depend on that in here, so it is possible that enabling this
+	  will have no effect.
+
 if KERNEL_KASAN
 	config KERNEL_KASAN_GENERIC
 	def_bool y