Input: sparse-keymap - implement safer freeing of the keymap

Allow calling sparse_keymap_free() before unregistering input device
whithout risk of racing with EVIOCGETKEYCODE and EVIOCSETKEYCODE.
This makes life of drivers writers easier.

Acked-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>