vdev_id: Support daisy-chained JBODs in multipath mode

Within function sas_handler() userspace commands like
'/usr/sbin/multipath' have been replaced with sourcing
device details from within sysfs which reduced a
significant amount of overhead and processing time.
Multiple JBOD enclosures and their order are sourced
from the bsg driver (/sys/class/enclosure) to isolate
chassis top-level expanders, which are then dynamically
indexed based on host channel of the multipath subordinate
disk member device being processed. Additionally added a
"mixed" mode for slot identification for environments where
a ZFS server system may contain SAS disk slots where there
is no expander (direct connect to HBA) while an attached
external JBOD with an expander have different slot identifier
methods.

How Has This Been Tested?
~~~~~~~~~~~~~~~~~~~~~~~~~

Testing was performed on a AMD EPYC based dual-server
high-availability multipath environment with multiple
HBAs per ZFS server and four SAS JBODs. The two primary
JBODs were multipath/cross-connected between the two
ZFS-HA servers. The secondary JBODs were daisy-chained
off of the primary JBODs using aligned SAS expander
channels (JBOD-0 expanderA--->JBOD-1 expanderA,
          JBOD-0 expanderB--->JBOD-1 expanderB, etc).
Pools were created, exported and re-imported, imported
globally with 'zpool import -a -d /dev/disk/by-vdev'.
Low level udev debug outputs were traced to isolate
and resolve errors.

Result:
~~~~~~~

Initial testing of a previous version of this change
showed how reliance on userspace utilities like
'/usr/sbin/multipath' and '/usr/bin/lsscsi' were
exacerbated by increasing numbers of disks and JBODs.
With four 60-disk SAS JBODs and 240 disks the time to
process a udevadm trigger was 3 minutes 30 seconds
during which nearly all CPU cores were above 80%
utilization. By switching reliance on userspace
utilities to sysfs in this version, the udevadm
trigger processing time was reduced to 12.2 seconds
and negligible CPU load.

This patch also fixes few shellcheck complains.

Reviewed-by: Gabriel A. Devenyi <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Co-authored-by: Jeff Johnson <[email protected]>
Signed-off-by: Jeff Johnson <[email protected]>
Signed-off-by: Arshad Hussain <[email protected]>
Closes #11526

Author: arshad512