IB: Correct MR length field to be 64-bit

The ib_mr->length represents the length of the MR in bytes as per
the IBTA spec 1.3 section 11.2.10.3 (REGISTER PHYSICAL MEMORY REGION).

Currently ib_mr->length field is defined as only 32-bits field.
This might result into truncation and failed WRs of consumers who
registers more than 4GB bytes memory regions and whose WRs accessing
such MRs.

This patch makes the length 64-bit to avoid such truncation.

Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Faisal Latif <faisal.latif@intel.com>
Fixes: 4c67e2bf

 ("IB/core: Introduce new fast registration API")
Signed-off-by: Ilya Lesokhin <ilyal@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>