lightnvm: pblk: initialize debug stat counter

[ Upstream commit a1121176ff757e3c073490a69608ea0b18a00ec1 ]

Initialize the stat counter for garbage collected reads.

Fixes: a4bd217b ("lightnvm: physical block device (pblk) target")
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>