scsi: core: Move scsi_host_busy() out of host lock for waking up EH handler

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

scsi: core: Move scsi_host_busy() out of host lock for waking up EH handler

Inside scsi_eh_wakeup(), scsi_host_busy() is called & checked with host

lock every time for deciding if error handler kthread needs to be waken up.

This can be too heavy in case of recovery, such as:

- N hardware queues

- queue depth is M for each hardware queue

- each scsi_host_busy() iterates over (N * M) tag/requests

If recovery is triggered in case that all requests are in-flight, each

scsi_eh_wakeup() is strictly serialized, when scsi_eh_wakeup() is called

for the last in-flight request, scsi_host_busy() has been run for (N * M -

1) times, and request has been iterated for (N*M - 1) * (N * M) times.

If both N and M are big enough, hard lockup can be triggered on acquiring

host lock, and it is observed on mpi3mr(128 hw queues, queue depth 8169).

Fix the issue by calling scsi_host_busy() outside the host lock. We don't

need the host lock for getting busy count because host the lock never

covers that.

[mkp: Drop unnecessary 'busy' variables pointed out by Bart]