RDMA/rxe: Cleanup page variables in rxe_mr.c

Cleanup usage of mr->page_shift and mr->page_mask and introduce
an extractor for mr->ibmr.page_size. Normal usage in the kernel
has page_mask masking out offset in page rather than masking out
the page number. The rxe driver had reversed that which was confusing.
Implicitly there can be a per mr page_size which was not uniformly
supported.

Link: https://lore.kernel.org/r/20230119235936.19728-6-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>