IB/mlx5, ib_post_send(), IB_WR_REG_SIG_MR: Do not modify the 'wr' argument

Since the next patch will constify the wr pointer, do not modify the data
that pointer points at.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>