bonding: set random address only when slaves already exist

After commit 5c3bf6cb ("bonding: assign random address if device
address is same as bond"), bonding will erroneously randomize the MAC
address of the first interface added to the bond if fail_over_mac =
follow.

Correct this by additionally testing for the bond being empty before
randomizing the MAC.

Fixes: 5c3bf6cb ("bonding: assign random address if device address is same as bond")
Reported-by: Qiuling Ren <qren@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250910024336.400253-1-liuhangbin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>