bonding: do not set usable_slaves for broadcast mode

bonding: do not set usable_slaves for broadcast mode

After commit e0caeb24f538 ("net: bonding: update the slave array for broadcast mode"),
broadcast mode will also set all_slaves and usable_slaves during
bond_enslave(). But if we also set updelay, during enslave, the
slave init state will be BOND_LINK_BACK. And later
bond_update_slave_arr() will alloc usable_slaves but add nothing.
This will cause bond_miimon_inspect() to have ignore_updelay
always true. So the updelay will be always ignored. e.g.

[    6.498368] bond0: (slave veth2): link status definitely down, disabling slave
[    7.536371] bond0: (slave veth2): link status up, enabling it in 0 ms
[    7.536402] bond0: (slave veth2): link status definitely up, 10000 Mbps full duplex

To fix it, we can either always call bond_update_slave_arr() on every
place when link changes. Or, let's just not set usable_slaves for
broadcast mode.

Fixes: e0caeb24f538 ("net: bonding: update the slave array for broadcast mode")
Reported-by: Liang Li <liali@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20260304-b4-bond_updelay-v1-1-f72eb2e454d0@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>