IPoIB/cm: Factor out ipoib_cm_create_srq()

Factor out the code to create an SRQ and allocate the receive ring in
ipoib_cm_dev_init() into a new function ipoib_cm_create_srq().  This
will make the code neater when support for devices that don't implement
SRQs is added.

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