blkcg: make blkcg_policy methods take a pointer to blkcg_policy_data

blkcg: make blkcg_policy methods take a pointer to blkcg_policy_data

The newly added ->pd_alloc_fn() and ->pd_free_fn() deal with pd
(blkg_policy_data) while the older ones use blkg (blkcg_gq).  As using
blkg doesn't make sense for ->pd_alloc_fn() and after allocation pd
can always be mapped to blkg and given that these are policy-specific
methods, it makes sense to converge on pd.

This patch makes all methods deal with pd instead of blkg.  Most
conversions are trivial.  In blk-cgroup.c, a couple method invocation
sites now test whether pd exists instead of policy state for
consistency.  This shouldn't cause any behavioral differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>