Fix bad metadata buffer assignations introduced when refactoring the
medatada write path.

Fixes: dd2a4343 lightnvm: pblk: sched. metadata on write thread
Signed-off-by: NJavier González <javier@cnexlabs.com>
Signed-off-by: NMatias Bjørling <matias@cnexlabs.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>