dm: unexport dm_get_queue_limits()

There are no dm_get_queue_limits() callers outside of DM core and
there shouldn't be.

Also, remove its BUG_ON(!atomic_read(&md->holders)) to micro-optimize
__process_abnormal_io().

Signed-off-by: Mike Snitzer <snitzer@kernel.org>

include/linux/device-mapper.h

@@ -530,8 +530,6 @@ int __init dm_early_create(struct dm_ioctl *dmi,
 			   struct dm_target_spec **spec_array,
 			   char **target_params_array);
 
-struct queue_limits *dm_get_queue_limits(struct mapped_device *md);
-
 /*
  * Geometry functions.
  */