nvme-rdma: default MR page size to 4k

Due to various page sizes in the system (IOMMU/device/kernel), we
set the fabrics controller page size to 4k and block layer boundaries
accordinglly. In architectures that uses different kernel page size
we'll have a mismatch to the MR page size that may cause a mapping error.
Update the MR page size to correspond to the core ctrl settings.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>