lightnvm: changed type of 'phys_sector' field in request struct.

Changed type of 'phys_sector' field in 'struct request' to
sector_t - to keep in line with the type for '__sector' after
which the field is modeled.
nvme: add compile-time size check of lnvm rw command struct

NVMe commands are of fixed size, this adds a compile-time check to
ensure that 'nvme_lnvm_rw_command is neither larger nor smaller
than a regular NVMe SQ command.