bonding: remove last users of bond->lock and bond->lock itself

The usage of bond->lock in bond_main.c was completely unnecessary as it
didn't help to sync with anything, most of the spots already had RTNL.
Since there're no more users of bond->lock, remove it.

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