RDMA/rxe: Move mw cleanup code to rxe_mw_cleanup()

Move code from rxe_dealloc_mw() to rxe_mw_cleanup() to allow flows which
hold a reference to mw to complete.

Link: https://lore.kernel.org/r/20220421014042.26985-7-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>