RDMA/rxe: Split MEM into MR and MW

RDMA/rxe: Split MEM into MR and MW

In the original rxe implementation it was intended to use a common object
to represent MRs and MWs but they are different enough to separate these
into two objects.

This allows replacing the mem name with mr for MRs which is more
consistent with the style for the other objects and less likely to be
confusing. This is a long patch that mostly changes mem to mr where it
makes sense and adds a new rxe_mw struct.

Link: https://lore.kernel.org/r/20210325212425.2792-1-rpearson@hpe.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Acked-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>