RDS/IB: print string constants in more places

This prints the constant identifier for work completion status and rdma
cm event types, like we already do for IB event types.

A core string array helper is added that each string type uses.

Signed-off-by: Zach Brown <zach.brown@oracle.com>

net/rds/rds.h

@@ -566,6 +566,7 @@ struct rds_statistics {
 };
 
 /* af_rds.c */
+char *rds_str_array(char **array, size_t elements, size_t index);
 void rds_sock_addref(struct rds_sock *rs);
 void rds_sock_put(struct rds_sock *rs);
 void rds_wake_sk_sleep(struct rds_sock *rs);