Fix a race in rxe_mcast.c that occurs when two QPs try at the same time to
attach a multicast address. Both QPs lookup the mgid address in a pool of
multicast groups and if they do not find it create a new group elem.

Fix this by locking the lookup/alloc/add key sequence and using the
unlocked APIs added in this patch set.

Link: https://lore.kernel.org/r/20201216231550.27224-8-rpearson@hpe.comSigned-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>