lightnvm: make nvm_set_rqd_ppalist() aware of vblks

A virtual block enables a block to identify multiple physical blocks.
This is useful for metadata where a device media supports multiple
planes. In that case, a block, with multiple planes can be managed
as a single vblk. Reducing the metadata required by one forth.

nvm_set_rqd_ppalist() takes care of expanding a ppa_list with vblks
automatically. However, for some use-cases, where only a single physical
block is required, the ppa_list should not be expanded.

Therefore, add a vblk parameter to nvm_set_rqd_ppalist(), and only
expand the ppa_list if vblk is set.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>