Combine copy_from_user() from create_user_qp() and general code.

Link: https://lore.kernel.org/r/20200427154636.381474-24-leon@kernel.orgReviewed-by: NMaor Gottlieb <maorg@mellanox.com>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>