rds: free ib_device related resource

rds: free ib_device related resource

There is a (rare) case that a ib_device gets removed(driver unload) while
upper layer(RDS) is still having references to the resources allocated
from this ib_device.

The result is either causing memory leak or crashing when accessing
the freed memory.

The resources are mainly rds_ib_mr objects, in-use rds_ib_mr (rds_mr)
objects are stored in rds_sock.rs_rdma_keys.

The fix is to
1) links up all in-use rds_ib_mr objects to the pool
2) links the rds_sock to rds_ib_mr
3) the destroy of the rds_ib_mr_pool takes care of freeing rds_ib_mrs
   by calling rds_rdma_drop_keys()

Orabug: 21795824

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Acked-by: Chien Yen <chien.yen@oracle.com>
Signed-off-by: Guangyu Sun <guangyu.sun@oracle.com>
Signed-off-by: Mukesh Kacker <mukesh.kacker@oracle.com>