CD burning broken again (crash) on 5.4-STABLE

[Sean McNeil]

Not sure when this broke again, but I am getting a panic when I try to
use nautilus cd burner again:

May  3 23:45:05 server kernel: Fatal trap 9: general protection fault while in kernel mode
May  3 23:45:05 server kernel: instruction pointer      = 0x8:0xffffffff801fb919
May  3 23:45:05 server kernel: stack pointer            = 0x10:0xffffffffb1b71b80
May  3 23:45:05 server kernel: frame pointer            = 0x10:0xffffffffb1b71bb0
May  3 23:45:05 server kernel: code segment             = base 0x0, limit 0xfffff, type 0x1b
May  3 23:45:05 server kernel: = DPL 0, pres 1, long 1, def32 0, gran 1
May  3 23:45:05 server kernel: processor eflags = interrupt enabled, resume, IOPL = 0
May  3 23:45:05 server kernel: current process          = 6 (thread taskq)
May  3 23:45:05 server kernel: trap number              = 9
May  3 23:45:05 server kernel: panic: general protection fault
May  3 23:45:05 server kernel: KDB: enter: panic
May  3 23:45:05 server kernel:
...

kernel.debug shows:

(gdb) l *0xffffffff801fb919
0xffffffff801fb919 is in ata_completed (/usr/src/sys/dev/ata/ata-queue.c:401).
396
397         ATA_DEBUG_RQ(request, "completed callback/wakeup");
398
399         /* get results back to the initiator */
400         if (request->callback)
401             (request->callback)(request);
402         else
403             sema_post(&request->done);
404
405         ata_start(ch);