net: ks8851: Reinstate disabling of BHs around IRQ handler

In the Linux kernel, the following vulnerability has been resolved:

net: ks8851: Reinstate disabling of BHs around IRQ handler

If the driver executes ks8851_irq() AND a TX packet has been sent, then
the driver enables TX queue via netif_wake_queue() which schedules TX
softirq to queue packets for this device.

If CONFIG_PREEMPT_RT=y is set AND a packet has also been received by
the MAC, then ks8851_rx_pkts() calls netdev_alloc_skb_ip_align() to
allocate SKBs for the received packets. If netdev_alloc_skb_ip_align()
is called with BH enabled, then local_bh_enable() at the end of
netdev_alloc_skb_ip_align() will trigger the pending softirq processing,
which may ultimately call the .xmit callback ks8851_start_xmit_par().
The ks8851_start_xmit_par() will try to lock struct ks8851_net_par
.lock spinlock, which is already locked by ks8851_irq() from which
ks8851_start_xmit_par() was called. This leads to a deadlock, which
is reported by the kernel, including a trace listed below.

If CONFIG_PREEMPT_RT is not set, then since commit 0913ec336a6c0
("net: ks8851: Fix deadlock with the SPI chip variant") the deadlock
can also be triggered without received packet in the RX FIFO. The
pending softirqs will be processed on return from
spin_unlock_bh(&ks->statelock) in ks8851_irq(), which triggers the
deadlock as well.

Fix the problem by disabling BH around critical sections, including the
IRQ handler, thus preventing the net_tx_action() softirq from triggering
during these critical sections. The net_tx_action() softirq is triggered
once BH are re-enabled and at the end of the IRQ handler, once all the
other IRQ handler actions have been completed.

__schedule from schedule_rtlock+0x1c/0x34
schedule_rtlock from rtlock_slowlock_locked+0x548/0x904
rtlock_slowlock_locked from rt_spin_lock+0x60/0x9c
rt_spin_lock from ks8851_start_xmit_par+0x74/0x1a8
ks8851_start_xmit_par from netdev_start_xmit+0x20/0x44
netdev_start_xmit from dev_hard_start_xmit+0xd0/0x188
dev_hard_start_xmit from sch_direct_xmit+0xb8/0x25c
sch_direct_xmit from __qdisc_run+0x1f8/0x4ec
__qdisc_run from qdisc_run+0x1c/0x28
qdisc_run from net_tx_action+0x1f0/0x268
net_tx_action from handle_softirqs+0x1a4/0x270
handle_softirqs from __local_bh_enable_ip+0xcc/0xe0
__local_bh_enable_ip from __alloc_skb+0xd8/0x128
__alloc_skb from __netdev_alloc_skb+0x3c/0x19c
__netdev_alloc_skb from ks8851_irq+0x388/0x4d4
ks8851_irq from irq_thread_fn+0x24/0x64
irq_thread_fn from irq_thread+0x178/0x28c
irq_thread from kthread+0x12c/0x138
kthread from ret_from_fork+0x14/0x28