Input: uinput - support injecting multiple events in one write() call

Rework the code in uinput_inject_event so that it matches the code in
evdev_write and allows injecting more than one event, or zero events.

Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>