checkpatch: add test for keywords not starting on tabstops

It's somewhat common and in general a defect for c90 keywords to
not start on a tabstop.

Add a test for this condition and warn when it occurs.
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2755,6 +2755,19 @@ sub process {
 			    "Logical continuations should be on the previous line\n" . $hereprev);
 		}
 
+# check indentation starts on a tab stop
+		if ($^V && $^V ge 5.10.0 &&
+		    $sline =~ /^\+\t+( +)(?:$c90_Keywords\b|\{\s*$|\}\s*(?:else\b|while\b|\s*$))/) {
+			my $indent = length($1);
+			if ($indent % 8) {
+				if (WARN("TABSTOP",
+					 "Statements should start on a tabstop\n" . $herecurr) &&
+				    $fix) {
+					$fixed[$fixlinenr] =~ s@(^\+\t+) +@$1 . "\t" x ($indent/8)@e;
+				}
+			}
+		}
+
 # check multi-line statement indentation matches previous line
 		if ($^V && $^V ge 5.10.0 &&
 		    $prevline =~ /^\+([ \t]*)((?:$c90_Keywords(?:\s+if)\s*)|(?:$Declare\s*)?(?:$Ident|\(\s*\*\s*$Ident\s*\))\s*|$Ident\s*=\s*$Ident\s*)\(.*(\&\&|\|\||,)\s*$/) {