bonding: rebuild the bond_resend_igmp_join_requests_delayed()

The bond_resend_igmp_join_requests_delayed() and
bond_resend_igmp_join_requests() should be integrated,
because the bond_resend_igmp_join_requests_delayed() did
nothing except bond_resend_igmp_join_requests().

The bond igmp_retrans could only be changed in bond_change_active_slave
and here, bond_change_active_slave will be called in RTNL and curr_slave_lock,
the bond_resend_igmp_join_requests already hold RTNL, so no need
to free RTNL and hold curr_slave_lock again, it may be a small optimization,
so move the igmp_retrans in RTNL and remove the curr_slave_lock.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>