docs: scripts/kernel-doc: Detect absence of FILE arg

Currently, when there is no FILE argument following a switch such
as -man, -rst, or -none, kernel-doc exits with a warning from perl
(long msg folded):

    Use of uninitialized value $ARGV[0] in pattern match (m//)
    at ./scripts/kernel-doc line 438.

, which is unhelpful.

Improve the behavior by adding a check at the bottom of parsing
loop.
If the argument is absent, display help text and exit with
the code of 1 (via usage()).

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/7b136049-a3ba-0eb5-8717-364d773ff914@gmail.com
[jc: reworked to fix conflict with pod patches]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -343,14 +343,23 @@ while ($ARGV[0] =~ m/^--?(.*)/) {
 	    die "Sphinx version should either major.minor or major.minor.patch format\n";
 	}
     } else {
-		# Unknown argument
+	# Unknown argument
-		pod2usage(
+	pod2usage(
-			-message => "Argument unknown!\n",
+	    -message => "Argument unknown!\n",
-			-exitval => 1,
+	    -exitval => 1,
-			-verbose => 99,
+	    -verbose => 99,
-			-sections => 'SYNOPSIS',
+	    -sections => 'SYNOPSIS',
-			-output => \*STDERR,
+	    -output => \*STDERR,
-		);
+	    );
+    }
+    if ($#ARGV < 0){
+	pod2usage(
+	    -message => "FILE argument missing\n",
+	    -exitval => 1,
+	    -verbose => 99,
+	    -sections => 'SYNOPSIS',
+	    -output => \*STDERR,
+	    );
     }
 }