RDS/IB: protect the list of IB devices

The RDS IB device list wasn't protected by any locking.  Traversal in
both the get_mr and FMR flushing paths could race with additon and
removal.

List manipulation is done with RCU primatives and is protected by the
write side of a rwsem.  The list traversal in the get_mr fast path is
protected by a rcu read critical section.  The FMR list traversal is
more problematic because it can block while traversing the list.  We
protect this with the read side of the rwsem.

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

net/rds/ib.h

@@ -23,6 +23,7 @@
 
 #define RDS_IB_RECYCLE_BATCH_COUNT	32
 
+extern struct rw_semaphore rds_ib_devices_lock;
 extern struct list_head rds_ib_devices;
 
 /*