lightnvm: use flags in rrpc_get_blk

rrpc_get_blk use constant 0 as the input parameter
of nvm_get_blk, this may result in getting gc block
failed unexpectedly.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>