SCSI EH wakeup deadlock from deferred_qc

[Igor Pylypiv <ipylypiv@google.com>]

Hi Damien,

I've stumbled upon an issue where SCSI EH didn't run upon command timeout
but ran ~10 or so seconds later. It seems like a recent regression from
the introduction of deffered_qc.

When an active command times out while a non-NCQ command is waiting in
deferred_qc, SCSI EH fails to wake up. Recovery stalls until the deferred
command's own timer expires.

When an NCQ command times out, scsi_timeout() calls scsi_eh_scmd_add(),
incrementing shost->host_failed (1). However, when scsi_eh_wakeup() checks
whether to wake the EH thread, scsi_host_busy(shost) counts 2 active
commands (1 timed-out + 1 in deferred_qc).

Because busy (2) != host_failed (1), scsi_eh_wakeup() refuses to wake
the EH thread, deadlocking error recovery until the deferred command
times out on its own.

Thanks,
Igor