Input: feed more data into entropy pool

Commit 4369c64c

 ("Input: Send events one packet at a time")
significantly reduced amount of entropy input core was feeding to the rest
of the system, because only the very first event in the event block would
be used as source of entropy.

With this change we will be calling add_input_randomness() for every event
that is not filtered by the input core as a duplicate. In addition, all
EV_SYN events are ignored.

Acked-by: Henrik Rydberg <rydberg@bitmath.org>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>