IB/core: Rename write flag to exclusive in rdma_core

We rename the "write" flags to "exclusive", as it's used for both
WRITE and DESTROY actions.

Fixes: 38321256 ('IB/core: Add support for idr types')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>