rds: Fix build regression.

Use DECLARE_* not DEFINE_*

Fixes: 8360ed67 ("RDS: IB: Use DEFINE_PER_CPU_SHARED_ALIGNED for rds_ib_stats")
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rds/ib.h

@@ -443,7 +443,7 @@ int rds_ib_send_grab_credits(struct rds_ib_connection *ic, u32 wanted,
 int rds_ib_xmit_atomic(struct rds_connection *conn, struct rm_atomic_op *op);
 
 /* ib_stats.c */
-DEFINE_PER_CPU_SHARED_ALIGNED(struct rds_ib_statistics, rds_ib_stats);
+DECLARE_PER_CPU_SHARED_ALIGNED(struct rds_ib_statistics, rds_ib_stats);
 #define rds_ib_stats_inc(member) rds_stats_inc_which(rds_ib_stats, member)
 #define rds_ib_stats_add(member, count) \
 		rds_stats_add_which(rds_ib_stats, member, count)