bonding: extract the use of real_device into local variable

Add a local variable for slave->dev, to prepare for the lock change in
the next patch. There is no functionality change.

Fixes: 9a560550 ("bonding: Add struct bond_ipesc to manage SA")
Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Jay Vosburgh <jv@jvosburgh.net>
Link: https://patch.msgid.link/20240823031056.110999-3-jianbol@nvidia.com


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