lib/ubsan: remove null-pointer checks

With gcc-8 fsanitize=null become very noisy.  GCC started to complain
about things like &a->b, where 'a' is NULL pointer.  There is no NULL
dereference, we just calculate address to struct member.  It's
technically undefined behavior so UBSAN is correct to report it.  But as
long as there is no real NULL-dereference, I think, we should be fine.

-fno-delete-null-pointer-checks compiler flag should protect us from any
consequences.  So let's just no use -fsanitize=null as it's not useful
for us.  If there is a real NULL-deref we will see crash.  Even if
userspace mapped something at NULL (root can do this), with things like
SMAP should catch the issue.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrey Ryabinin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/Kconfig.ubsan

@@ -1,9 +1,6 @@
 config ARCH_HAS_UBSAN_SANITIZE_ALL
 	bool
 
-config ARCH_WANTS_UBSAN_NO_NULL
-	def_bool n
-
 config UBSAN
 	bool "Undefined behaviour sanity checker"
 	help
@@ -39,14 +36,6 @@ config UBSAN_ALIGNMENT
 	  Enabling this option on architectures that support unaligned
 	  accesses may produce a lot of false positives.
 
-config UBSAN_NULL
-	bool "Enable checking of null pointers"
-	depends on UBSAN
-	default y if !ARCH_WANTS_UBSAN_NO_NULL
-	help
-	  This option enables detection of memory accesses via a
-	  null pointer.
-
 config TEST_UBSAN
 	tristate "Module for testing for undefined behavior detection"
 	depends on m && UBSAN