CVE-2025-71132: smc91x: fix broken irq-context in PREEMPT_RT

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

smc91x: fix broken irq-context in PREEMPTRT

When smc91x.c is built with PREEMPTRT, the following splat occurs in FVPRevC:

[ 13.055000] smc91x LNRO0003:00 eth0: link up, 10Mbps, half-duplex, lpa 0x0000 [ 13.062137] BUG: workqueue leaked atomic, lock or RCU: kworker/2:1[106] [ 13.062137] preempt=0x00000000 lock=0->0 RCU=0->1 workfn=mldifcwork [ 13.062266] C replaying previous printk message [ 13.062266] CPU: 2 UID: 0 PID: 106 Comm: kworker/2:1 Not tainted 6.18.0-dirty #179 PREEMPT{RT,(full)} [ 13.062353] Hardware name: , BIOS [ 13.062382] Workqueue: mld mldifcwork [ 13.062469] Call trace: [ 13.062494] showstack+0x24/0x40 (C) [ 13.062602] dumpstack+0x28/0x48 [ 13.062710] dumpstacklvl+0x7c/0xb0 [ 13.062818] dumpstack+0x18/0x34 [ 13.062926] processscheduledworks+0x294/0x450 [ 13.063043] workerthread+0x260/0x3d8 [ 13.063124] kthread+0x1c4/0x228 [ 13.063235] retfromfork+0x10/0x20

This happens because smcspecialtrylock() disables IRQs even on PREEMPTRT, but smcspecialunlock() does not restore IRQs on PREEMPTRT. The reason is that smcspecialunlock() calls spinunlockirqrestore(), and rcureadunlockbh() in devqueuexmit() cannot invoke rcureadunlock() through localbhenableip() when current->softirqdisablecnt becomes zero.

To address this issue, replace smcspecialtrylock() with spintrylockirqsave().