IB/core: Use qp->usecnt to track multicast attach/detach

Just as we don't allow PDs, CQs, etc. to be destroyed if there are QPs
that are attached to them, don't let a QP be destroyed if there are
multicast group(s) attached to it.  Use the existing usecnt field of
struct ib_qp which was added by commit 0e0ec7e0 ("RDMA/core: Export
ib_open_qp() to share XRC TGT QPs") to track this.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>