IPoIB/cm: Factor out ipoib_cm_free_rx_reap_list()

Factor out the code for going through the rx_reap list of struct
ipoib_cm_rx and freeing each one.  This consolidates the code
duplicated between ipoib_cm_dev_stop() and ipoib_cm_rx_reap() and
reduces the risk of error when adding additional accounting.

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