IPoIB/cm: Use vmalloc() to allocate rx_rings

There are users that are running UDP applications that require a large
receive queue size in order to get good performance.  To prevent
allocation failures for rx_rings when using non-SRQ mode and large
recv_queue_size (1K or larger), use vmalloc() instead of kcalloc() to
alocate rx_rings.

Signed-off-by: David Wilder <dwilder@us.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>