checkpatch: add --strict "pointer comparison to NULL" test

It seems there are more and more uses of "if (!ptr)" in preference to "if
(ptr == NULL)" so add a --strict test to emit a message when using the
latter form.

This also finds (ptr != NULL).

Fix it too if desired.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -4522,6 +4522,20 @@ sub process {
 			     "Possible precedence defect with mask then right shift - may need parentheses\n" . $herecurr);
 		}
 
+# check for pointer comparisons to NULL
+		if ($^V && $^V ge 5.10.0) {
+			while ($line =~ /\b$LvalOrFunc\s*(==|\!=)\s*NULL\b/g) {
+				my $val = $1;
+				my $equal = "!";
+				$equal = "" if ($4 eq "!=");
+				if (CHK("COMPARISON_TO_NULL",
+					"Comparison to NULL could be written \"${equal}${val}\"\n" . $herecurr) &&
+					    $fix) {
+					$fixed[$fixlinenr] =~ s/\b\Q$val\E\s*(?:==|\!=)\s*NULL\b/$equal$val/;
+				}
+			}
+		}
+
 # check for bad placement of section $InitAttribute (e.g.: __initdata)
 		if ($line =~ /(\b$InitAttribute\b)/) {
 			my $attr = $1;