Input: prepare for f_ops constness

Avoid doing assignments to a live ->fops so it can be marked as 'const'.

Signed-off-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>