checkpatch: add ability to find bad uses of vsprintf %p<foo> extensions

%pK was at least once misused at %pk in an out-of-tree module.  This
lead to some security concerns.  Add the ability to track single and
multiple line statements for misuses of %p<foo>.

[akpm@linux-foundation.org: add helpful comment into lib/vsprintf.c]
[akpm@linux-foundation.org: text tweak]
Link: http://lkml.kernel.org/r/163a690510e636a23187c0dc9caa09ddac6d4cde.1488228427.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: William Roberts <william.c.roberts@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/vsprintf.c

@@ -1477,6 +1477,9 @@ int kptr_restrict __read_mostly;
  * by an extra set of alphanumeric characters that are extended format
  * specifiers.
  *
+ * Please update scripts/checkpatch.pl when adding/removing conversion
+ * characters.  (Search for "check for vsprintf extension").
+ *
  * Right now we handle:
  *
  * - 'F' For symbolic function descriptor pointers with offset