RDMA/rxe: Use members of generic struct in rxe_mr

rxe_mr and ib_mr have interchangeable members. Remove device specific
members and use ones in the generic struct. Both 'iova' and 'length' are
filled in ib_uverbs or ib_core layer after MR registration.

Signed-off-by: Daisuke Matsuda <matsuda-daisuke@fujitsu.com>
Link: https://lore.kernel.org/r/20220921080844.1616883-2-matsuda-daisuke@fujitsu.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>