checkpatch: fix multi-statement macro checks for while blocks.

Checkpatch.pl doesn't have a check for excluding while (...) {...} blocks
from MULTISTATEMENT_MACRO_USE_DO_WHILE error.

For example, running checkpatch.pl on the file mm/maccess.c in the kernel
generates the following error:

ERROR: Macros with complex values should be enclosed in parentheses
+#define copy_from_kernel_nofault_loop(dst, src, len, type, err_label)  \
+       while (len >= sizeof(type)) {                                   \
+               __get_kernel_nofault(dst, src, type, err_label);        \
+               dst += sizeof(type);                                    \
+               src += sizeof(type);                                    \
+               len -= sizeof(type);                                    \
+       }

The error is misleading for this case.  Enclosing it in parentheses
doesn't make any sense.

Checkpatch already has an exception list for such common macro types.
Added a new exception for while (...) {...} style blocks to the same.

In addition, the brace flatten logic was modified by changing the
substitution characters from "1" to "1u".  This was done to ensure that
macros in the form "#define foo(bar) while(bar){bar--;}" were also
correctly procecssed.

Link: https://lore.kernel.org/linux-kernel-mentees/dc985938aa3986702815a0bd68dfca8a03c85447.camel@perches.com/Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20201001171903.312021-1-dwaipayanray1@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -5351,9 +5351,9 @@ sub process {
 			$dstat =~ s/\s*$//s;
 
 			# Flatten any parentheses and braces
-			while ($dstat =~ s/\([^\(\)]*\)/1/ ||
-			       $dstat =~ s/\{[^\{\}]*\}/1/ ||
-			       $dstat =~ s/.\[[^\[\]]*\]/1/)
+			while ($dstat =~ s/\([^\(\)]*\)/1u/ ||
+			       $dstat =~ s/\{[^\{\}]*\}/1u/ ||
+			       $dstat =~ s/.\[[^\[\]]*\]/1u/)
 			{
 			}
 
@@ -5394,6 +5394,7 @@ sub process {
 			    $dstat !~ /^\.$Ident\s*=/ &&				# .foo =
 			    $dstat !~ /^(?:\#\s*$Ident|\#\s*$Constant)\s*$/ &&		# stringification #foo
 			    $dstat !~ /^do\s*$Constant\s*while\s*$Constant;?$/ &&	# do {...} while (...); // do {...} while (...)
+			    $dstat !~ /^while\s*$Constant\s*$Constant\s*$/ &&		# while (...) {...}
 			    $dstat !~ /^for\s*$Constant$/ &&				# for (...)
 			    $dstat !~ /^for\s*$Constant\s+(?:$Ident|-?$Constant)$/ &&	# for (...) bar()
 			    $dstat !~ /^do\s*{/ &&					# do {...