[$] Supporting multi-actuator drives

In a combined filesystem and storage session at the 2018 Linux Storage,Filesystem, and Memory-Management Summit (LSFMM), Tim Walker asked for helpin designing the interface to some new storage hardware. He wanted somefeedback on how a multi-actuatordrive should present itself to the system. These drives have two (or, eventually, more) sets of read/write heads andother hardware that can all operate in parallel.