Add a fallback to the device mapper logic. We've seen systems in the field
that are apparently misconfigured by the manufacturer and cause the mapping
logic to fail. The fallback allows drive numbers to be assigned based on the
PHY number that they're attached to. Add sysctls and tunables to overrid
this new behavior, but they should be considered only necessary for debugging.

Reviewed by: imp, smh
Obtained from: Netflix
MFC after: 3 days
Sponsored by: D8403