lightnvm: pblk: fix write I/O sync stat

Fix stat counter to collect the right number of I/Os being synced on the
completion path.

Fixes: 0880a9aa

 ("lightnvm: pblk: delete redundant buffer pointer")
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>