kbuild/headers_check: refine extern check

'extern' checking information is not clear, refine it.
Plus, fix a comment.

Signed-off-by: WANG Cong <amwang@redhat.com>
[sam: redid the extern error message]
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/headers_check.pl

@@ -2,7 +2,7 @@
 #
 # headers_check.pl execute a number of trivial consistency checks
 #
-# Usage: headers_check.pl dir [files...]
+# Usage: headers_check.pl dir arch [files...]
 # dir:   dir to look for included files
 # arch:  architecture
 # files: list of files to check
@@ -37,7 +37,7 @@ foreach my $file (@files) {
 		&check_include();
 		&check_asm_types();
 		&check_sizetypes();
-		&check_prototypes();
+		&check_declarations();
 		# Dropped for now. Too much noise &check_config();
 	}
 	close FH;
@@ -61,10 +61,12 @@ sub check_include
 	}
 }
 
-sub check_prototypes
+sub check_declarations
 {
-	if ($line =~ m/^\s*extern\b/) {
-		printf STDERR "$filename:$lineno: extern's make no sense in userspace\n";
+	if ($line =~m/^\s*extern\b/) {
+		printf STDERR "$filename:$lineno: " .
+		              "userspace cannot call function or variable " .
+		              "defined in the kernel\n";
 	}
 }