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

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

Correct the function parameters to eliminate kernel-doc warnings:

blk-cgroup-rwstat.h:63: warning: Function parameter or struct member 'opf' not described in 'blkg_rwstat_add'
blk-cgroup-rwstat.h:63: warning: Excess function parameter 'op' description in 'blkg_rwstat_add'
blk-cgroup-rwstat.h:91: warning: Function parameter or struct member 'result' not described in 'blkg_rwstat_read'

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/20250111062748.910442-1-rdunlap@infradead.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>