RDS/IB: disconnect when IB devices are removed

Currently IB device removal destroys connections which are associated with the
device.  This prevents connections from being re-established when replacement
devices are added.

Instead we'll queue shutdown work on the connections as their devices are
removed.  When we see that devices are added we triger connection attempts on
all connections that don't currently have a device.

The result is that RDS sockets can resume device-independent work (bcopy, not
RDMA) across IB device removal and restoration.

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