RDMA/mlx5: Add missing synchronize_srcu() for MW cases

While MR uses live as the SRCU 'update', the MW case uses the xarray
directly, xa_erase() causes the MW to become inaccessible to the pagefault
thread.

Thus whenever a MW is removed from the xarray we must synchronize_srcu()
before freeing it.

This must be done before freeing the mkey as re-use of the mkey while the
pagefault thread is using the stale mkey is undesirable.

Add the missing synchronizes to MW and DEVX indirect mkey and delete the
bogus protection against double destroy in mlx5_core_destroy_mkey()

Fixes: 534fd7aac5 ("IB/mlx5: Manage indirection mkey upon DEVX flow for ODP")
Fixes: 6aec21f6a8 ("IB/mlx5: Page faults handling infrastructure")
Link: https://lore.kernel.org/r/20191001153821.23621-7-jgg@ziepe.ca
Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/mlx5_ib.h

@@ -639,7 +639,6 @@ struct mlx5_ib_mw {
 struct mlx5_ib_devx_mr {
 	struct mlx5_core_mkey	mmkey;
 	int			ndescs;
-	struct rcu_head		rcu;
 };
 
 struct mlx5_ib_umr_context {