IB/mlx5: Fix incorrect size of klms in the memory region

The value of mr->ndescs greater than mr->max_descs is set in the
function mlx5_ib_sg_to_klms() if sg_nents is greater than
mr->max_descs. This is an invalid value and it causes the
following error when registering mr:

mlx5_0:dump_cqe:276:(pid 193): dump error cqe
00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00000030: 00 00 00 00 0f 00 78 06 25 00 00 8b 08 1e 8f d3

Cc: <stable@vger.kernel.org> # 4.5
Fixes: b005d316 ("mlx5: Add arbitrary sg list support")
Signed-off-by: Sergey Gorenko <sergeygo@mellanox.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>