checkpatch: add warning for uses of printk_ratelimit

Warn about uses of printk_ratelimit() because it uses a global state and
can hide subsequent useful messages.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>