Change the common code to use already copied user command buffer.

Link: https://lore.kernel.org/r/20200427154636.381474-12-leon@kernel.orgReviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>