Replace int num_qp in struct rxe_mcg by atomic_t qp_num.

Link: https://lore.kernel.org/r/20220208211644.123457-5-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>