nouveau/u_memcpya: use vmemdup_user

I think there are limit checks in place for most things but the
new uAPI wants to not have them.

Add a limit check and use the vmemdup_user helper instead.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230810185020.231135-1-airlied@gmail.com