blk-cgroup: fix freeing NULL blkg in blkg_create

new_blkg can be NULL if the caller didn't pass in a pre-allocated blkg.
Don't try to free it in that case.

Fixes: 27b642b0 ("blk-cgroup: simplify blkg freeing from initialization failure paths")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20230206150201.3438972-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>