staging: vchiq: Move copy callback handling into vchiq

staging: vchiq: Move copy callback handling into vchiq

All vchi users use the kernel variant of the copy callback. The only
user for the user space variant of the copy callback is in the ioctl
implementation. So move all this copying logic into vchiq, and expose a
new function that explicitly passes kernel messages.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200527115400.31391-4-nsaenzjulienne@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>