Initialize the stat counter for garbage collected reads.

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: NJavier González <javier@cnexlabs.com>
Signed-off-by: NMatias Bjørling <m@bjorling.me>
Signed-off-by: NJens Axboe <axboe@kernel.dk>