Documentation/sparse: add hints about __CHECKER__

Several attributes depend on __CHECKER__, but previously there was no
clue in the tree about when __CHECKER__ might be defined.  Add hints at
the most common places (__kernel, __user, __iomem, __bitwise) and in the
sparse documentation.

Link: https://lkml.kernel.org/r/20220310220927.245704-3-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Michael S . Tsirkin" <mst@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/sparse.rst

@@ -100,3 +100,5 @@ have already built it.
 
 The optional make variable CF can be used to pass arguments to sparse.  The
 build system passes -Wbitwise to sparse automatically.
+
+Note that sparse defines the __CHECKER__ preprocessor symbol.

include/linux/compiler_types.h

@@ -4,6 +4,7 @@
 
 #ifndef __ASSEMBLY__
 
+/* sparse defines __CHECKER__; see Documentation/dev-tools/sparse.rst */
 #ifdef __CHECKER__
 /* address spaces */
 # define __kernel	__attribute__((address_space(0)))

include/uapi/linux/types.h

@@ -19,6 +19,7 @@
  * any application/library that wants linux/types.h.
  */
 
+/* sparse defines __CHECKER__; see Documentation/dev-tools/sparse.rst */
 #ifdef __CHECKER__
 #define __bitwise	__attribute__((bitwise))
 #else