checkpatch.pl: warn on duplicate sysctl local variable

Commit d91bff3 ("proc/sysctl: add shared variables for range
check") adds some shared const variables to be used instead of a local
copy in each source file.  Warn when a chunk duplicates one of these
values in a ctl_table struct:

    $ scripts/checkpatch.pl 0001-test-commit.patch
    WARNING: duplicated sysctl range checking value 'zero', consider using the shared one in include/linux/sysctl.h
    #27: FILE: arch/arm/kernel/isa.c:48:
    +               .extra1         = &zero,

    WARNING: duplicated sysctl range checking value 'int_max', consider using the shared one in include/linux/sysctl.h
    #28: FILE: arch/arm/kernel/isa.c:49:
    +               .extra2         = &int_max,

    total: 0 errors, 2 warnings, 14 lines checked

Link: http://lkml.kernel.org/r/20190531131422.14970-1-mcroce@redhat.com
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Joe Perches <joe@perches.com>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -6639,6 +6639,12 @@ sub process {
 				     "unknown module license " . $extracted_string . "\n" . $herecurr);
 			}
 		}
+
+# check for sysctl duplicate constants
+		if ($line =~ /\.extra[12]\s*=\s*&(zero|one|int_max)\b/) {
+			WARN("DUPLICATED_SYSCTL_CONST",
+				"duplicated sysctl range checking value '$1', consider using the shared one in include/linux/sysctl.h\n" . $herecurr);
+		}
 	}
 
 	# If we have no input at all, then there is nothing to report on