staging: vchiq: convert compat dequeue_message

Split out the ioctl implementation for VCHIQ_IOC_DEQUEUE_MESSAGE
into a separate function so it can be shared with the compat
implementation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200918095441.1446041-4-arnd@arndb.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>