scsi: target: tcmu: Adjust names of variables and definitions

Some definitions and members of struct tcmu_dev had misleading
names. Examples:

 - ring_size was used for the size of mailbox + cmd ring + data area

 - CMDR_SIZE was used for size of mailbox + cmd ring

I added the new definition MB_CMDR_SIZE (mailbox + command ring), changed
CMDR_SIZE to hold the size of the command ring only and replaced in struct
tcmu_dev the member ring_size with mmap_pages, because the member is now
used in tcmu_mmap() only, where we need page count, not size.

I also added the new struct tcmu_dev member 'cmdr' which is used to replace
some occurences of '(void *)mb + CMDR_OFF' with 'udev->cmdr' for better
readability.

Link: https://lore.kernel.org/r/20210324195758.2021-2-bostroesser@gmail.com
Signed-off-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>