IPoIB/cm: Factor out ipoib_cm_free_rx_ring()

Factor out the code to unmap/free skbs and free the receive ring in
ipoib_cm_dev_cleanup() into a new function ipoib_cm_free_rx_ring().
This function will be called from a couple of other places when
support for devices that don't implement SRQs is added.

Signed-off-by: Roland Dreier <rolandd@cisco.com>