'extern' checking information is not clear, refine it.
Plus, fix a comment.
Signed-off-by: NWANG Cong <amwang@redhat.com>
[sam: redid the extern error message]
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>