tty: Use find_next_bit helper in tty_verify_active_pairs

Since we have find_next_bit in our sources now, lets
use for_each_bit helper to speedup a search procedure.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>