bug.h: Add WARN_RATELIMIT

Add a generic mechanism to ratelimit WARN(foo, fmt, ...) messages
using a hidden per call site static struct ratelimit_state.

Also add an __WARN_RATELIMIT variant to be able to use a specific
struct ratelimit_state.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>