IB/ehca: MR/MW structure refactoring

- Rename struct ehca_mr fields to clearly distinguish between kernel
  and HW page size.
- Sort struct ehca_mr_pginfo into a common part and a union containing
  specific fields for physical, user and fast MR

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>