Create a new sysctl and tunable to help ensure sane numbering of devices.
There are cases where the device_mapper code fails to generating mappings
between device handles and target numbers, or persistent mappings create
unpredictable holes in the numbering. This knob allows the driver to
fall back to a simple phy number mapping if the device mapper fails, or
allows the operator to override the device mapper entirely.
1 - use the PhyNum field as a fallback to the mapping logic
0 - never use the PhyNum field
-1 - only use the PhyNum field