block: todo to fix error handling for lightnvm map

Insert todo. Make sure command is aborted if map fails.
lightnvm: rename nvm_queue_rq to nvm_map_rq

The nvm_queue_rq no longer acts as a querying mechanism within the block
layer. It simply maps a given logical address into the physical block(s)
and leaves it up to the device driver to pick up the work.

Also remove the queue return semantics.
lightnvm: implement logical to physical device mapping

To enable both the logical and physical address to be communicated to
the device, we extend the structure with a phys_sector, that can be used
be the device driver.

For the nvme driver, we overwrite dword 14 and 15 of the NVMe rw
command. This overwrites the end-to-end protection, but allows us to
directly communicate the physical address in a single command to the
device, without using an metadata buffer.

This fixes the rq->__sector hack that we have had applied beforehand.