bonding: fix data-races around agg_select_timer

commit 9ceaf6f7 upstream.

syzbot reported that two threads might write over agg_select_timer
at the same time. Make agg_select_timer atomic to fix the races.

BUG: KCSAN: data-race in bond_3ad_initiate_agg_selection / bond_3ad_state_machine_handler

read to 0xffff8881242aea90 of 4 bytes by task 1846 on cpu 1:
 bond_3ad_state_machine_handler+0x99/0x2810 drivers/net/bonding/bond_3ad.c:2317
 process_one_work+0x3f6/0x960 kernel/workqueue.c:2307
 worker_thread+0x616/0xa70 kernel/workqueue.c:2454
 kthread+0x1bf/0x1e0 kernel/kthread.c:377
 ret_from_fork+0x1f/0x30

write to 0xffff8881242aea90 of 4 bytes by task 25910 on cpu 0:
 bond_3ad_initiate_agg_selection+0x18/0x30 drivers/net/bonding/bond_3ad.c:1998
 bond_open+0x658/0x6f0 drivers/net/bonding/bond_main.c:3967
 __dev_open+0x274/0x3a0 net/core/dev.c:1407
 dev_open+0x54/0x190 net/core/dev.c:1443
 bond_enslave+0xcef/0x3000 drivers/net/bonding/bond_main.c:1937
 do_set_master net/core/...