blk-cgroup: fix kernel-doc warnings in header file

blk-cgroup: fix kernel-doc warnings in header file

Correct the function parameters and function names to eliminate
kernel-doc warnings:

blk-cgroup.h:238: warning: Function parameter or struct member 'bio' not described in 'bio_issue_as_root_blkg'
blk-cgroup.h:248: warning: bad line:
blk-cgroup.h:279: warning: expecting prototype for blkg_to_pdata(). Prototype was for blkg_to_pd() instead
blk-cgroup.h:296: warning: expecting prototype for pdata_to_blkg(). Prototype was for pd_to_blkg() instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: cgroups@vger.kernel.org
Link: https://lore.kernel.org/r/20250111062736.910383-1-rdunlap@infradead.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>