scsi: sr: get rid of sr global mutex

This is a commit of the version that predates
upstreamed commit 51a8588

When replacing the Big Kernel Lock in commit
2a48fc0 ("block: autoconvert trivial
BKL users to private mutex"), the lock was replaced with a sr-wide lock.

This causes very poor performance when using multiple sr devices, as the
sr driver was not able to execute more than one command to one drive at
any given time, even when there were many CD drives available.

Replace the global mutex with per-sr-device mutex.

Someone tried this patch at the time, but it never made it
upstream, due to possible concerns with race conditions, but it's not
clear the patch actually caused those:

https://www.spinics.net/lists/linux-scsi/msg63706.html
https://www.spinics.net/lists/linux-scsi/msg63750.html

Also see

http://lists.xiph.org/pipermail/paranoia/2019-December/001647.html