RDMA/rxe: Fix bug rejecting all multicast packets

Fix a bug in rxe_rcv() that causes all multicast packets to be
dropped. Currently rxe_match_dgid() is called for each packet to verify
that the destination IP address matches one of the entries in the port
source GID table. This is incorrect for IP multicast addresses since they
do not appear in the GID table.

Add code to detect multicast addresses.

Change function name to rxe_chk_dgid() which is clearer.

Link: https://lore.kernel.org/r/20201008212753.265249-1-rpearson@hpe.com


Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>