bonding: allow xmit hash policy change while bond dev is up

Since the xmit_hash_policy pointer is always valid and not dependent on
anything, we can change it while the bond device is up and running. The
only downside would be the out of order packets but that is a small price
to pay.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>