bonding: add missed __rcu annotation for curr_active_slave

There is one direct accesses to bond->curr_active_slave in
bond_miimon_commit(). Protected it by rcu_access_pointer()
since the later of this function also use this one.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>