IB/cm: Remove cma_multicast->igmp_joined

IB/cm: Remove cma_multicast->igmp_joined

This variable isn't read and written to with proper locking, so it is
racy. Instead of using an unlocked bool use presence in the mc->list

The caller could race rdma_join_multicast with rdma_leave_multicast which
would leak a mc join and cause a use after free of mc.

Instead, do not add the mc to the list until it has completed
initialization, all mcs on the list require leaving.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>