RDMA/rxe: Protect user space index loads/stores

RDMA/rxe: Protect user space index loads/stores

Modify the queue APIs to protect all user space index loads with
smp_load_acquire() and all user space index stores with
smp_store_release(). Base this on the types of the queues which can be one
of ..KERNEL, ..FROM_USER, ..TO_USER. Kernel space indices are protected by
locks which also provide memory barriers.

Link: https://lore.kernel.org/r/20210527194748.662636-3-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>