block: fix double-free in the failure path of cgwb_bdi_init()

block: fix double-free in the failure path of cgwb_bdi_init()

When !CONFIG_CGROUP_WRITEBACK, bdi has single bdi_writeback_congested
at bdi->wb_congested.  cgwb_bdi_init() allocates it with kzalloc() and
doesn't do further initialization.  This usually works fine as the
reference count gets bumped to 1 by wb_init() and the put from
wb_exit() releases it.

However, when wb_init() fails, it puts the wb base ref automatically
freeing the wb and the explicit kfree() in cgwb_bdi_init() error path
ends up trying to free the same pointer the second time causing a
double-free.

Fix it by explicitly initilizing the refcnt to 1 and putting the base
ref from cgwb_bdi_destroy().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: a13f35e87140 ("writeback: don't embed root bdi_writeback_congested in bdi_writeback")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Jens Axboe <axboe@fb.com>