checkpatch: warn on uses of __constant_<foo> functions

Emit a warning when using any of these __constant_<foo> forms:

	__constant_cpu_to_be[x]
	__constant_cpu_to_le[x]
	__constant_be[x]_to_cpu
	__constant_le[x]_to_cpu
	__constant_htons
	__constant_ntohs

Using any of these outside of include/uapi/ isn't preferred as using the
function without __constant_ is identical when the argument is a
constant.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -3931,6 +3931,19 @@ sub process {
 			}
 		}
 
+# don't use __constant_<foo> functions outside of include/uapi/
+		if ($realfile !~ m@^include/uapi/@ &&
+		    $line =~ /(__constant_(?:htons|ntohs|[bl]e(?:16|32|64)_to_cpu|cpu_to_[bl]e(?:16|32|64)))\s*\(/) {
+			my $constant_func = $1;
+			my $func = $constant_func;
+			$func =~ s/^__constant_//;
+			if (WARN("CONSTANT_CONVERSION",
+				 "$constant_func should be $func\n" . $herecurr) &&
+			    $fix) {
+				$fixed[$linenr - 1] =~ s/\b$constant_func\b/$func/g;
+			}
+		}
+
 # prefer usleep_range over udelay
 		if ($line =~ /\budelay\s*\(\s*(\d+)\s*\)/) {
 			my $delay = $1;