checkpatch: Fix whitespace checks for documentation code blocks

Prevent blank lines in documentation code blocks to be signalled as
incorrect trailing whitespace.

Code blocks in documentation are 4-column aligned, and blank lines in
them should have exactly 4 columns of trailing whitespace to prevent
QEMU's wiki to render them as separate code blocks.
Signed-off-by: NLluís Vilanova <vilanova@ac.upc.edu>
Message-Id: <147325254382.22644.5531276787733455773.stgit@fimbulvetr.bsc.es>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NLluís Vilanova <vilanova@ac.upc.edu>

scripts/checkpatch.pl

@@ -1320,6 +1320,16 @@ sub process {
 			my $herevet = "$here\n" . cat_vet($rawline) . "\n";
 			ERROR("DOS line endings\n" . $herevet);
 
+		} elsif ($realfile =~ /^docs\/.+\.txt/ ||
+			 $realfile =~ /^docs\/.+\.md/) {
+		    if ($rawline =~ /^\+\s+$/ && $rawline !~ /^\+ {4}$/) {
+			# TODO: properly check we're in a code block
+			#       (surrounding text is 4-column aligned)
+			my $herevet = "$here\n" . cat_vet($rawline) . "\n";
+			ERROR("code blocks in documentation should have " .
+			      "empty lines with exactly 4 columns of " .
+			      "whitespace\n" . $herevet);
+		    }
 		} elsif ($rawline =~ /^\+.*\S\s+$/ || $rawline =~ /^\+\s+$/) {
 			my $herevet = "$here\n" . cat_vet($rawline) . "\n";
 			ERROR("trailing whitespace\n" . $herevet);