scsi: scsi_debug: Add no_rwlock parameter

scsi: scsi_debug: Add no_rwlock parameter

By default, this driver places a read lock around all user data fetches and
a write lock around all user data modifying operations (e.g. WRITE
commands). These locks have "per store" granularity.  Other drivers that
have a similar function (e.g. null_blk) do not take this data integrity
step and run significantly faster in some tests.

In the common case of a (simulated) device to device copy (e.g.  what dd
and its variants do) there should be no need for locks around data
accesses. So add the driver and sysfs parameter no_rwlock which is boolean
and when set does what its name suggests. The default is false for backward
comaptibility.

Link: https://lore.kernel.org/r/20220109012853.301953-7-dgilbert@interlog.com
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>