scripts: kernel-doc: fix attribute capture in function parsing

Currently, kernel-doc warns for function prototype parsing on the
presence of attributes "__attribute_const__" and "__flatten" in the
definition.

There are 166 occurrences in ~70 files in the kernel tree for
"__attribute_const__" and 5 occurrences in 4 files for "__flatten".

Out of 166, there are 3 occurrences in three different files with
"__attribute_const__" and a preceding kernel-doc; and, 1 occurrence in
./mm/percpu.c for "__flatten" with a preceding kernel-doc. All other
occurrences have no preceding kernel-doc.

Add support for  "__attribute_const__" and "__flatten" attributes.

A quick evaluation by running 'kernel-doc -none' on kernel-tree reveals
that no additional warning or error has been added or removed by the fix.
Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
Link: https://lore.kernel.org/r/20210306113510.31023-1-yashsri421@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -1766,12 +1766,14 @@ sub dump_function($$) {
     $prototype =~ s/^noinline +//;
     $prototype =~ s/__init +//;
     $prototype =~ s/__init_or_module +//;
+    $prototype =~ s/__flatten +//;
     $prototype =~ s/__meminit +//;
     $prototype =~ s/__must_check +//;
     $prototype =~ s/__weak +//;
     $prototype =~ s/__sched +//;
     $prototype =~ s/__printf\s*\(\s*\d*\s*,\s*\d*\s*\) +//;
     my $define = $prototype =~ s/^#\s*define\s+//; #ak added
+    $prototype =~ s/__attribute_const__ +//;
     $prototype =~ s/__attribute__\s*\(\(
             (?:
                  [\w\s]++          # attribute name