scsi: target: tcmu: Make cmd_ring_size changeable via configfs

Make cmd_ring_size changeable similar to the way it is done for
max_data_area_mb. The reason is that our tcmu client will create thousands
of tcmu instances, and this will consume lots of mem with default 8Mb cmd
ring size for every backstore.

One can change the value by typing:

    echo "cmd_ring_size_mb=N" > control

The "N" is a integer between 1 to 8, if set 1, the cmd ring can hold about
6k cmds(tcmu_cmd_entry about 176 byte) at least.

The value is printed when doing:

    cat info

In addition, a new readonly attribute 'cmd_ring_size_mb' returns the value
in read.

Link: https://lore.kernel.org/r/1644978109-14885-1-git-send-email-kanie@linux.alibaba.com


Reviewed-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>