RDMA/cma: Correct detection of SA Created MGID

RDMA CM treats AF_INET6 addresses that are either 0 or prefixed with
FF1x:A01B::/32 as MGIDs, but the detection for the prefix was buggy;
fix it up.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>