RDMA/cma: Merge cma_get/save_net_info

With the removal of SDP related code, we can merge cma_get_net_info()
with cma_save_net_info(), since we're only ever dealing with a single
header format.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>