block: export bio_associate_*() and wbc_account_io()

bio_associate_blkcg(), bio_associate_current() and wbc_account_io()
are used to implement cgroup writeback support for filesystems and
thus need to be exported.  Export them.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <axboe@fb.com>