IB/mlx5: Compare only index part of a memory window rkey

The InfiniBand Architecture Specification section 10.6.7.2.4 TYPE 2 MEMORY
WINDOWS says that if the CI supports the Base Memory Management Extensions
defined in this specification, the R_Key format for a Type 2 Memory Window
must consist of:

* 24 bit index in the most significant bits of the R_Key, which is owned
  by the CI, and
* 8 bit key in the least significant bits of the R_Key, which is owned by
  the Consumer.

This means that the kernel should compare only the index part of a R_Key
to determine equality with another R_Key.

Fixes: db570d7d

 ("IB/mlx5: Add ODP support to MW")
Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>