bonding: Rename slave_arr to usable_slaves

[ Upstream commit ed7d4f023b1a9b0578f20d66557c66452ab845ec ]

Rename slave_arr to usable_slaves, since we will have two arrays,
one for the usable slaves and the other to all slaves.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Stable-dep-of: f8a65ab2f3ff ("bonding: fix link recovery in mode 2 when updelay is nonzero")
Signed-off-by: Sasha Levin <sashal@kernel.org>