Input: mousedev - fix memory leak

Apparently, "while true; do cat </dev/null >/dev/input/mice; done" causes
an OOM in a short amount of time. Funny that nobody noticed, it actually
is very easy to trigger just by switching between VT1 and VT7...

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>