RDMA/cma: Optimize cma_bind_loopback() to check for empty list

Optimize to test for an empty list first.  This ends up simplifying
the code too.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>