rds_message_alloc
rm = rds_message_alloc(extra_bytes, GFP_NOWAIT);
struct rds_message *rds_message_alloc(unsigned int nents, gfp_t gfp);
rm = rds_message_alloc(ret, GFP_KERNEL);
rm = rds_message_alloc(0, GFP_ATOMIC);