Input: sysrq - rework re-inject logic

Internally 'disable' the filter when re-injecting Alt-SysRq instead
of relying on input core to suppress delivery of injected events
to the originating handler.

This allows to revert commit 5fdbe44d


which causes problems with existing userspace programs trying to
loopback the events via evdev.

Reported-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>