batman-adv: generalise tt_local_reset_flags()

The tt_local_reset_flags() is actually used for one use case only. It is not
generalised enough to be used indifferent situations. This patch make it general
enough in order to let other code use it whenever a flag set is requested over
the whole hash table (passed as parameter). The function is now called
tt_set_flags()

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>