checkpatch: check for if's with unnecessary parentheses

If statements don't need multiple parentheses around tested comparisons
like "if ((foo == bar))".

An == comparison maybe a sign of an intended assignment, so emit a
slightly different message if so.
Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -3254,6 +3254,20 @@ sub process {
 			}
 		}
 
+# if statements using unnecessary parentheses - ie: if ((foo == bar))
+		if ($^V && $^V ge 5.10.0 &&
+		    $line =~ /\bif\s*((?:\(\s*){2,})/) {
+			my $openparens = $1;
+			my $count = $openparens =~ tr@\(@\(@;
+			my $msg = "";
+			if ($line =~ /\bif\s*(?:\(\s*){$count,$count}$LvalOrFunc\s*($Compare)\s*$LvalOrFunc(?:\s*\)){$count,$count}/) {
+				my $comp = $4;	#Not $1 because of $LvalOrFunc
+				$msg = " - maybe == should be = ?" if ($comp eq "==");
+				WARN("UNNECESSARY_PARENTHESES",
+				     "Unnecessary parentheses$msg\n" . $herecurr);
+			}
+		}
+
 # Return of what appears to be an errno should normally be -'ve
 		if ($line =~ /^.\s*return\s*(E[A-Z]*)\s*;/) {
 			my $name = $1;