bonding: adjust locking comments

Now that locks have been removed, remove some unnecessary comments and
adjust others to reflect reality. Also add a comment to "mode_lock" to
describe its current users and give a brief summary why they need it.

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