staging: vchiq: convert compat await_completion

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

This one is the trickiest conversion, as the compat implementation
is already quite different from the native one. By using a common
handler, the behavior is changed to be the same again: The
indirect __user pointer accesses are now handled through helper
functions that check for compat mode internally.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200918095441.1446041-6-arnd@arndb.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>