Input: sysrq - pass along lone Alt + SysRq

When user presses and releases Alt + SysRq without pressing any of the
hot keys re-inject the combination and pass it on to userspace instead
of suppressing it - maybe he or she wanted to take print screen
instead of invoking SysRq handler.

Also pass along release events for keys that have been pressed before
SysRq mode has been invoked so that keys do not appear to be "stuck".

Acked-by: Jason Wessel <jason.wessel@windriver.com>
Tested-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>