In bond priority testing, we set the primary interface to eth1 and add
eth0,1,2 to bond in serial. This is OK in normal times. But when in
debug kernel, the bridge port that eth0,1,2 connected would start
slowly (enter blocking, forwarding state), which caused the primary
interface down for a while after enslaving and active slave changed.
Here is a test log from Jakub's debug test[1].

 [  400.399070][   T50] br0: port 1(s0) entered disabled state
 [  400.400168][   T50] br0: port 4(s2) entered disabled state
 [  400.941504][ T2791] bond0: (slave eth0): making interface the new active one
 [  400.942603][ T2791] bond0: (slave eth0): Enslaving as an active interface with an up link
 [  400.943633][ T2766] br0: port 1(s0) entered blocking state
 [  400.944119][ T2766] br0: port 1(s0) entered forwarding state
 [  401.128792][ T2792] bond0: (slave eth1): making interface the new active one
 [  401.130771][ T2792] bond0: (slave eth1): Enslaving as an active interface with an up link
 [  401.131643][   T69] br0: port 2(s1) entered blocking state
 [  401.132067][   T69] br0: port 2(s1) entered forwarding state
 [  401.346201][ T2793] bond0: (slave eth2): Enslaving as a backup interface with an up link
 [  401.348414][   T50] br0: port 4(s2) entered blocking state
 [  401.348857][   T50] br0: port 4(s2) entered forwarding state
 [  401.519669][  T250] bond0: (slave eth0): link status definitely down, disabling slave
 [  401.526522][  T250] bond0: (slave eth1): link status definitely down, disabling slave
 [  401.526986][  T250] bond0: (slave eth2): making interface the new active one
 [  401.629470][  T250] bond0: (slave eth0): link status definitely up
 [  401.630089][  T250] bond0: (slave eth1): link status definitely up
 [...]
 # TEST: prio (active-backup ns_ip6_target primary_reselect 1)         [FAIL]
 # Current active slave is eth2 but not eth1

Fix it by setting active slave to primary slave specifically before
testing.

[1] https://netdev-3.bots.linux.dev/vmksft-bonding-dbg/results/464301/1-bond-options-sh/stdout



Fixes: 481b56e0 ("selftests: bonding: re-format bond option tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>