rds: rcu-ize rds_ib_get_device()

rds_ib_get_device is called very often as we turn an
ip address into a corresponding device structure.  It currently
take a global spinlock as it walks different lists to find active
devices.

This commit changes the lists over to RCU, which isn't very complex
because they are not updated very often at all.

Signed-off-by: Chris Mason <chris.mason@oracle.com>