checkpatch: Detect '%#' or '%0#' in printf-style format strings

According to the coding style document, we should use literal '0x' prefix
instead of printf's '#' flag (which appears as '%#' or '%0#' in the format
string).  Add a checkpatch rule to enforce that.

Note that checkpatch already had a similar rule for trace-events files.

Example usage:

  $ scripts/checkpatch.pl --file chardev/baum.c
  ...
  ERROR: Don't use '#' flag of printf format ('%#') in format strings, use '0x' prefix instead
  #366: FILE: chardev/baum.c:366:
  +            DPRINTF("Broken packet %#2x, tossing\n", req); \
  ...
  ERROR: Don't use '#' flag of printf format ('%#') in format strings, use '0x' prefix instead
  #472: FILE: chardev/baum.c:472:
  +        DPRINTF("unrecognized request %0#2x\n", req);
  ...

Signed-off-by: Dov Murik <dovmurik@linux.vnet.ibm.com>
Message-Id: <20200914172623.72955-1-dovmurik@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

scripts/checkpatch.pl

@@ -2880,14 +2880,20 @@ sub process {
 				$herecurr);
 		}
 
-# check for %L{u,d,i} in strings
+# format strings checks
 		my $string;
 		while ($line =~ /(?:^|")([X\t]*)(?:"|$)/g) {
 			$string = substr($rawline, $-[1], $+[1] - $-[1]);
 			$string =~ s/%%/__/g;
+			# check for %L{u,d,i} in strings
 			if ($string =~ /(?<!%)%L[udi]/) {
 				ERROR("\%Ld/%Lu are not-standard C, use %lld/%llu\n" . $herecurr);
-				last;
+			}
+			# check for %# or %0# in printf-style format strings
+			if ($string =~ /(?<!%)%0?#/) {
+				ERROR("Don't use '#' flag of printf format " .
+				      "('%#') in format strings, use '0x' " .
+				      "prefix instead\n" . $herecurr);
 			}
 		}