The max_scsicmpl_time parameter can be used to perform scsi cmd queue
depth mgmt based on io completion time: the queue depth is reduced to
make completion time shorter. However, as soon as an io completes and
the completion time is within limits, the code immediately bumps the
queue depth limit back up to the target queue depth. Thus the procedure
restarts, effectively limiting the usefulness of adjusting queue depth
to help completion time.

This patch makes the following changes:

 - Removes the code at io completion that resets the queue depth as soon
   as within limits.

 - As the code removed was where the target queue depth was first
   applied, change target queue depth application so that it occurs when
   the parameter is changed.

 - Makes target queue depth a standard parameter: both a module
   parameter and a sysfs parameter.

 - Optimizes the command pending count by using atomics rather than
   locks.

 - Updates the debugfs nodelist stats to allow better debugging of
   pending command counts.
Signed-off-by: NDick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: NJames Smart <james.smart@broadcom.com>
Reviewed-by: NHannes Reinecke <hare@suse.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>