There's no point in calling radix_tree_preload() if preloading doesn't
use more permissible GFP mask.  Drop preloading from
__blkg_lookup_create().

While at it, drop sparse locking annotation which no longer applies.

v2: Vivek pointed out the odd preload usage.  Instead of updating,
    just drop it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NVivek Goyal <vgoyal@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>