Move index increment after its is used or otherwise it will start the dump
of the WQE from second WQE BB.

Fixes: 34f4c955 ("IB/mlx5: Use fragmented QP's buffer for in-kernel users")
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>