Staging: rdma: Use memdup_user

Use memdup_user when user data is immediately copied into the
allocated region.

Issue found using coccinelle.

Signed-off-by: Ksenija <Stanojevic&lt;ksenija.stanojevic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>