checkpatch: emit a warning on file add/move/delete

Whenever files are added, moved, or deleted, the MAINTAINERS file
patterns can be out of sync or outdated.

To try to keep MAINTAINERS more up-to-date, add a one-time warning
whenever a patch does any of those.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>