checkpatch: fix CONST_STRUCT when const_structs.checkpatch is missing

Checkpatch reports warnings when some specific structs are not declared as
const in the code.  The list of structs to consider was initially defined
in the checkpatch.pl script itself, but it was later moved to an external
file (scripts/const_structs.checkpatch), in commit bf1fa1da
("checkpatch: externalize the structs that should be const").  This
introduced two minor issues:

- When file scripts/const_structs.checkpatch is not present (for
  example, if checkpatch is run outside of the kernel directory with the
  "--no-tree" option), a warning is printed to stderr to tell the user
  that "No structs that should be const will be found". This is fair,
  but the warning is printed unconditionally, even if the option
  "--ignore CONST_STRUCT" is passed. In the latter case, we explicitly
  ask checkpatch to skip this check, so no warning should be printed.

- When scripts/const_structs.checkpatch is missing, or even when trying
  to silence the warning by adding an empty file, $const_structs is set
  to "", and the regex used for finding structs that should be const,
  "$line =~ /struct\s+($const_structs)(?!\s*\{)/)", matches all
  structs found in the code, thus reporting a number of false positives.

Let's fix the first item by skipping scripts/const_structs.checkpatch
processing if "CONST_STRUCT" checks are ignored, and the second one by
skipping the test if $const_structs is not defined. Since we modify the
read_words() function a little bit, update the checks for
$typedefsfile/$typeOtherTypedefs as well.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Joe Perches <joe@perches.com>
Link: http://lkml.kernel.org/r/20200623221822.3727-1-quentin@isovalent.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>