scsi: core: Ensure that the SCSI error handler gets woken up

If scsi_eh_scmd_add() is called concurrently with
scsi_host_queue_ready() while shost->host_blocked > 0 then it can
happen that neither function wakes up the SCSI error handler. Fix
this by making every function that decreases the host_busy counter
wake up the error handler if necessary and by protecting the
host_failed checks with the SCSI host lock.

Reported-by: Pavel Tikhomirov <[email protected]>
References: https://marc.info/?l=linux-kernel&m=150461610630736
Fixes: commit 746650160866 ("scsi: convert host_busy to atomic_t")
Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Pavel Tikhomirov <[email protected]>
Tested-by: Stuart Hayes <[email protected]>
Cc: Konstantin Khorenko <[email protected]>
Cc: Stuart Hayes <[email protected]>
Cc: Pavel Tikhomirov <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>

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