blkg_destroy() caches @blkg->q in local variable @q.  While there are
two places which needs @blkg->q, only lockdep_assert_held() used the
local variable leading to unused local variable warning if lockdep is
configured out.  Drop the local variable and just use @blkg->q
directly.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NRakesh Iyer <rni@google.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>