arm64: kconfig: Move LIST_POISON to a safe value

commit bf0c4e04 upstream.

Move the poison pointer offset to 0xdead000000000000, a
recognized value that is not mappable by user-space exploits.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Thierry Strudel <tstrudel@google.com>
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

arch/arm64/Kconfig

@@ -81,6 +81,10 @@ config NO_IOPORT_MAP
 config STACKTRACE_SUPPORT
 	def_bool y
 
+config ILLEGAL_POINTER_VALUE
+	hex
+	default 0xdead000000000000
+
 config LOCKDEP_SUPPORT
 	def_bool y